Can I update my iMac?

The age of the specific mac will govern if you can upgrade to a newer macOS

Go to the apple logo in the menu bar, tick that, and then tick about this mac. You can see your computer type here. Then, compare it with this article for monterey: macOS Monterey is compatible with these computers - Apple Support (SG)

This link is for Big Sur macOS Big Sur is compatible with these computers - Apple Support (SG)

and finally this link for Catalina macOS Catalina is compatible with these computers - Apple Support (SG)

If your computer cannot run these versions of MacOs there is a chance that it can Run High Sierra.

In principle, yes; though, just like any other operating system, let alone, hardware specifications, the upgrade options will range between "Beta" and "not eligible".

For clarity, by "Beta", I meant that if your device or software version is "handpicked" by the developer, you can "upgrade" it to the "soon-to-be-latest" version.

By "not eligible", of course, I meant that the situation is extreme whereby your version is the last and can no longer be updated.

I hope I'm able to help you in framing your direction as to when, how, and what to upgrade. :)

Regards,

RRB