After new battery installation how do I reset watch?

Hi, According to your problem first make sure that the battery that you have changed is OK, or try another battery. Your watch need a factory reset. If normal factory reseting doesn't solve your problem then you have to repair the machine.

Some watches have a "reset" button or combination of buttons that must be pushed to clear all the electronic registries on the watch's little computer after you replace the battery. You might try taking off the watch back and seeing if your watch has this requirement. Even then, it's not always a perfect solution. Last week, I replaced the battery in a watch that had been dead for some time. The watch appeared to come alive again when I put the new battery in, but when I pushed the reset pin (on this movement, it was on the watch movement itself, next to the battery), the entire watch went dead. Nothing I did would wake it up. I removed the battery, waited 30 seconds, then re-inserted the battery. Problem solved; the watch powered up, and I was able to set the time and access all functions again.
On some digital watches from the 1980s, after replacing the batteries, you were supposed to push and hold all the external buttons for a few seconds to reset the watch. I don't think manufacturers still make watches that respond to this, but it is easier to try than taking off the back and re-installing the battery.

When a new battery is installed it is often required to short the case of the watch to the chassis of the watch (using tweezers). This resets the watch. Some watches have "AC" inscribed at the point where the shorting should be done.

Help with battery problem on Timex Ironman Triathlon after installing a new battery and reset, displays the message 846010 1.0 P1and the alarm is silent.

Remove the four screws that hold the battery cover on the back of the watch. Remove the 3 very small screws that hold the battery in place. Remove the battery from the battery holder. The piece that the 3 small screws were removed from seperates from the 3 volt 2016 energizer battery. Replace the battery with a new one. Re-install the battery holder to the watch. There is a small tab on the lower left part of the battery holder that needs to be depressed to activate the watch. Depress it and look at the face of the watch to make sure it is activated. The watch will need to have the time, date etc reset but you should now be able to see that the battery is working. Replace the battery cover and reset the time and date as needed.

This reset trick works with other Casio Alarm Chrono digital watches, such as my old Model W-720, similar to your F91-W. After the new lithium coil cell is installed and secured with the spring clip, the watch may show random segments in the LCD display or may simply be blank. Insert a sewing needle, paper clip, or blade of a small jeweler's screwdriver into the hole on the back of the movement stamped "AC" (All Clear) to momentarily connect the gold plated contact at the bottom of the hole to the stainless steel battery clip. The battery clip in most such Casio watches is connected to the "+" terminal of the coin cell. This will reset the CMOS chip and it should start up with the watch displaying 12:00 and the date at 1-1. Then, use the control buttons to set the time and date in the normal fashion.

There is a reset button inside your watch, but I don't think that is the problem. Most likely, there is a spring that makes contact to the watch and the back of your watch case, and it fell out by the person replacing your battery. I would definitely go back to the place where you had the battery installed, and have them take care of the problem buy ordering you a new spring or have them pay for you to send it to Casio.