How can I replace cpu in my acer desktop

First thing you need to do is establish if your heatsink and cooling fan are doing their job. The heatsink MUST have paste to help transfer heat. The heatsink MUST be clean so air can flow. The fan must run properly and the BIOS must monitor the heat properly.

Learn all you can from books, magazine articles, forum entries, anywhere you can find information. Make sure you have all the info you need before tackling this project. You can do it; just learn all you can.