How to uniquely identify a process?

I have an application (let's call it Main.exe) and a helper process (call it Update.exe) that periodically updates Main app.

Update.exe needs to reside in memory, but I need to make sure that EXACTLY ONE process is running.

Anyway, other "Update.exe" processes may run simultaneously at one time, so I need to check the right one. What's the most reliable method?