Recovered Older iMac with Snow Leopard OSX

You will find a list of iMac computers here:

https://everymac.com/systems/apple/imac/index-imac.html

User guides can be found here:

Mac Manuals and Downloads - Apple Support

Information about how to replace a hard drive, et cetera can usually be found via a web search for something like the exact model + disassembly/replace.

Depending on which specific iMac you have, either Snow Leopard (10.6.*), El Capitan (10.11.*), or High Sierra (10.13.*) would be the end of the line.

Even in the best case, High Sierra is itself quite old.

Help you do what?

Snow Leopard is very old at this point. What do you intend to do with that Mac?