is there a way to update an iMac 20inch mid 2007?

It is to old to waste any time or money on to update the hardware. Plus it can only run up to 10.11 El Capitan, which Apple no longer provides security updates for.

Sadly, your best bet is to retire/recycle it and look into getting a new or at least a newer model Mac.

To what, it's 14 years old has zero value and has been considered obsolete for years. I think I'd upgrade it to being a fish tank or maybe an ant farm.