can high sierra be updated on my iMac?

To give you a precise answer, we need to know the SUB-MODEL of your iMac. Please do "About this Mac..." from your Apple menu and tell us what that shows for SUB-MODEL, the part with a year. Be sure to include any modifiers to the year like Early, Mid, or Late.

⚠️ DO NOT post the serial number for security reasons.

This is an example of the High Sierra version of the "About..." window showing where to find the sub-model:

Unless your iMac is Late 2012 or newer, it cannot run higher than High Sierra.

What year model is your iMac?

see > Identify your iMac model - Apple Support

Safari updates are only available if your iMac is compatible with later macOS updates.

see > How to download and install macOS - Apple Support

If your iMac is not capable of running a later macOS version than Catalina, then you need to use a third party browser like Mozilla FireFox, until you eventually replace the iMac with a new one. see > https://www.mozilla.org/en-US/firefox/mac/