I don't know how to update my Browser?

Also please tell us what macOS your computer is running. Even non-Safari browsers hace limits on how old a macOS they support.

Please tell us your iMac's sub-model from "About this Mac." With that we can advise you on your system upgrade options. Not all Macs can run the latest macOS versions.

Examples of where to find "sub-model" using "About this Mac":

Lacking such details form you, we can only offer generic responses that may not fully address your issue.

What browser? If it is Safari, it can only be updated when you upgrade your operating system because Safari is installed as part of the OS.