How to increase IMAC computer memory to stop getting alerts

This is the way: Install memory in an iMac - Apple Support

Describe the alert you are encountering. As Allan Jones alluded to it may be a scam.

It's very easy to determine if your Mac's performance is being limited by available memory. Please read Check if your Mac needs more RAM in Activity Monitor - Apple Support. If the "Memory Pressure" graph is frequently "red" your Mac will benefit from additional memory. If not, it won't. It's that simple.

Sometimes those "alerts" are due to useless anti-virus and so-called "cleaning" apps the user misguidedly installed. They interfere with built-in protection and maintenance routines you paid Apple to develop for macOS>

If you have any, remove them per their developers' instructions.