How do you change the number of decimal places?

If you mean it is not showing everything after the decimal point, you can change that by pressing the [f] key followed by the number of decimal places to display. For example, to restrict it to 2 places, press [f][2]. If you want to see 4 places, press [f][4].

To show all possible decimal places, use the sequence [f][9].
From the manual (http://www.hp.com/ctg/Manual/c00363319.pdf):
"To display a different number of decimal places, press f followed by a digit key (0 through 9) specifying the number of decimal places. In the following examples, notice how the displayed form of the number inside the calculator - 14.87456320 - is rounded to the specified number of digits."


"Although nine decimal places were
specified after f, only eight are
displayed since the display can
show a total of only 10 digits. "
The standard display format, plus the specified number of decimal places, remain in effect until you change them; they are not reset each time the calculator is turned on. However, if Continuous Memory is reset, when the calculator is next turned on numbers will be displayed in the standard display format with two decimal places
shown.
If a calculated answer is either too small or too large to be displayed in the standard display format, the display format automatically switches to scientific notation. The display returns to the standard display format for all numbers that can be displayed in that format.
http://www.hp.com/ctg/Manual/c00363319.pdf

Initial Setup

One adjustment is important before we get started. By default the 12C displays only two decimal places. This is not enough. Personally, I like to see five decimal places, but you may prefer some other number. To change the display, press "F" and then the number of decimal places you wish to display. I would press "F 5 " to display 5 decimal places. That's it, the calculator is ready to go.
You need to hold the "F" key.---------

1. Press and release the gold PREFIX key (f).
2. Press the number key for the desired number of decimal places (0 through 9).

------------ Click this link below and follow the procedure:--- http://howtobyme.blogspot.com/2011/07/hp-12c-calculator-help.html ----------- This should help.Thanks.Helpmech.

CUT: Values are cut off to the number of decimal places speicified by the current Decimal Place Selector setting

UP: Values are rounded up to the number of decimal places speicified by the current Decimal Place Selector setting

5/4: Values are rounded off (0,1,2,3,4 are cut off; 5,6,7,8,9 are rounded up) to the number of decimal places speicified by the current Decimal Place Selector setting

Press 2nd [FORMAT], type the number of digits you want after the decimal point or 9 for floating-decimal, then press ENTER 2nd [QUIT].

Decimal Point and Separator Symbols
You can use the display setup (Disp) screen to specify the symbols you want for the decimal point and 3-digit separator.
• To change the decimal point and separator symbol setting, press the MODE key a number of times until you reach the setup screen shown below.
Display the selection screen. (Disp) 1.
Press the number key ( 1 or 2) that corresponds to the setting you want to use.
1(Dot): Period decimal point, comma separator
2(Comma): Comma decimal point, period separator

pressing the tab button displays the number of decimal places 0, 2, 3, 4, A, F on the bottom of the screen.

Hello,
It seems to me that you are omitting a step: see screen capture from manual (step 4). Hope it helps.

Check this HP FAQ:
Setting a Fixed Decimal Place You can download the manual and user guides for your calculator from here:
http://h10025.www1.hp.com/ewfrf/wc/manualCategory?lc=en&dlc=en&cc=us&product=81575&lang=en&

For more information, Faqs & how-Tosgo the HP support homepage for your calculator:
http://h10025.www1.hp.com/ewfrf/wc/product?lc=en&dlc=en&cc=us&lang=en&product=81575&

Introduction This document describes how to set the desired number of decimal places on the HP 12c calculator in standard format. It also covers how to enter a number in scientific notation. Instructions for standard format

1. Press and release .
2. Press the number key for the desired number of decimal places to be displayed to the right of the decimal point (0 through 9). The calculator displays a maximum of 10 digits.

The display will show the specified number of decimal places. The answer will be rounded to the number of digits selected. Example (standard format) If the number in the display is 14.87456320, the display will be changed as follows: Keystroke Display 14.8746 14.9 15 14.87456320 Instructions for scientific notation Use scientific notation for very large or very small numbers. Type in the mantissa. After pressing, the next numbers keyed in are exponents of 10. Press . If during an operation a result has more than 10 digits, the calculator automatically displays the number in scientific notation

Introduction This document describes how to set the desired number of decimal places on the HP 12c calculator in standard format. It also covers how to enter a number in scientific notation. Instructions for standard format

1. Press and release .
2. Press the number key for the desired number of decimal places to be displayed to the right of the decimal point (0 through 9). The calculator displays a maximum of 10 digits.

The display will show the specified number of decimal places. The answer will be rounded to the number of digits selected. Example (standard format) If the number in the display is 14.87456320, the display will be changed as follows: Keystroke Display 14.8746 14.9 15 14.87456320 Instructions for scientific notation Use scientific notation for very large or very small numbers. Type in the mantissa. After pressing, the next numbers keyed in are exponents of 10. Press . If during an operation a result has more than 10 digits, the calculator automatically displays the number in scientific notation

Your BA II PLUS stores numeric values internally to an accuracy of 13 digits. However, the numbers it displays depends on the number of decimal places to which you have it set (choice of 0–9 or floating decimal). To set the number of decimal places displayed to three: hit [2ND][FORMAT][3][ENTER]This setting also continues indefinitely (even though the calculator is turned off and on) until it is changed.

Your BA II PLUS stores numeric values internally to an accuracy of 13 digits. However, the numbers it displays depends on the number of decimal places to which you have it set (choice of 0–9 or floating decimal). To set the number of decimal places displayed to three: hit [2ND][FORMAT][5][ENTER]This setting also continues indefinitely (even though the calculator is turned off and on) until it is changed.