I need step by step on how to upgrade cpu on my presario 7594 desktop ,windows 98SE,it came with 566mhz processor,I was told Ican go up tp 1.0 ghz, after I power up ,what is all I step i needed to do

I'm not sure I go along with the advice you were given.
Going to get a little detailed here, but the following info is necessary IMHO.

If you have any questions regarding the info, please state so in a Comment.

Supporting Data:
1) The Intel Celeron 566 processor you have installed now, has these specifications;

A) Operates at 566MegaHertz at full operating capability. (566MHz)

B) Uses a 66MHz Front Side Bus. (66MHz FSB)

C) Use the Socket 370 processor socket.

D) Operates on a DC voltage of 1.5 to 1.7 Volts

http://en.wikipedia.org/wiki/List_of_intel_celeron_microprocessors#.22Coppermine-128.22_.28180_nm.29

(Coppermine-128 Core. 180nm architecture.
Part Number RB80526RX566128.
sSpec number, SL46T, or SL3WT, or SL4NW, or SL5L5, or SL4PC.

The P/N and sSpec number, are on top of the processor's case. Seen after the Heatsink/Fan combo, and thermal paste is removed)

[Mega = approximately 1 Million.
Hertz stands for Cycles per Second.
Your processor when operating at maximum capability, has a frequency rate of 566 Million Cycles per Second]

The Front Side Bus requirement, and the Voltage requirement, are the two things that concern me.
Voltage more so.

To explain:
On the motherboard is a motherboard chipset.
Chip and Chipset, are slang terms for Integrated Circuit, or IC.

The motherboard chipset is the Northbridge chip, and the Southbridge chip.

The Northbridge chip is what detemines what Processors can be used, as well as Ram Memory, and high speed graphics.

(You only have 4 white PCI slots on that motherboard, and they are not used for high speed graphics. AGP, and PCI Express are high speed graphics. Just added info)

Intel Pentium III processors that use a Socket 370 processor socket, and operate at 1000MHz, use a 100Mhz FSB.

It is my belief that the Northbridge chipset on your motherboard, will support a processor with either a 66MHz FSB, or 100MHz FSB, so this is not an issue.

What concerns me most is that Intel Pentium processors using a Socket 370 processor socket, and having a maximum frequency rate of 1000Mhz, require 1.75 volts.

No if's, and's, or but's, about it.
Has to be 1.75 volts, and not up to 1.7 volts.

I would think that an Intel Pentium III, that uses a Socket 370 processor socket, and operates at 850MHz, is more suitable.

http://en.wikipedia.org/wiki/List_of_Intel_Pentium_III_microprocessors#.22Coppermine.22_.28180_nm.29

(Coppermine Core. 180nm architecture.
sSpec numbers SL4MC, or SL43H, or SL49G, or SL4CC, or SL4Z2)

Like this example,

http://www.weirdstuff.com/cgi-bin/item/13386

(Not advertising for this website. Will state I have purchased a motherboard from them, and it is like they stated)

To replace your Processor:

1) Work on a table. Computer UNPLUGGED from power. (Not shouting) Open the computer case, and touch the metal frame with your fingers.

Your body carries Static electricity. Static will fry out (Short Circuit) the delicate hardware components inside a computer.

Touching the metal frame of the computer case will relieve your body of Static.

IF, you get up, and walk away in the middle of working on your computer, when you come back, TOUCH the metal frame BEFORE you touch any computer components, or reach inside your computer.

Processors are the most highly susceptible computer hardware component, to Static shock.
You can also buy an ESD wrist strap, and connect the alligator clip to the metal frame of the computer case.
Cost is around $6.

Materials needed to include the replacement Processor.

A) Thermal Paste:
You can buy a small tube for a one time application at a lot of computer repair shops, for around $2 to $3.
Online, or at stores that sell computer parts, the price averages $6 to $8.

Imperative! You must have fresh thermal paste, and you must use it.

B) Isopropyl Alcohol. Also called Rubbing Alcohol.
Will be used to clean the old thermal paste off of the bottom of the Heatsink, and also to insure that the top of the replacement Processor, is clean.

91 percent alcohol is the best, but 70 percent will suffice. Advise do Not use 50-50. (50 percent alcohol, 50 percent water)

BE ADVISED!
Isopropyl Alcohol is Extremely Flammable!
Use in a well ventilated area, with no sparks, or flames present.

C) Q-tips. Used with the alcohol to clean. Q-tips are dipped in the alcohol. May take many of them, as old thermal paste is usually gooey.

D) An old plastic credit card, or similar device. Used to scrape the old thermal paste off, before the alcohol is used.

Will detail more in an additional comment.

You should replace the unit with a new one. Most new software today requires at least 1Ghz processor, recommended 2 Ghz.

Also, this is not a software update. You must open the box and replace the CPU on the mainboard with another compatible one that uses the socket 370 FC-PGA.

Unless this unit is absolutely necessary for business like running machinery, it should be replaced outright as you can get better hardware for just a few hundred dollars.