how do I find app_config.xml ?

I think it is:

right click on computer icon desktop

this PC - -> OS C - -> Program Files - -> BOINC --> then ?

Very close...

this PC - -> OS C - -> ProgramData - -> BOINC --> projects --> www.primegrid.com

Note that you may need to change your Windows Explorer settings to even be able to see ProgramData since it is hidden by default. Also note there's no space between Program and Data.

Please heed the part about "if you don't know what this is, you don't need to do anything."

Once the new system goes live (probably in about two weeks) everything you're going to do with app_config will likely be rendered obsolete. My recommendation is just to wait until the new system goes live, and then use the new web-based controls.