if my mac is running on version 10.12, how do i update it?

From menu : About This Mac, look at the product description line including the parenthetical date info. A Mac that can run Sierra (10.12.6) can also upgrade to macOS High Sierra (10.13.6), and perhaps further depending upon hardware compatibility confirmed in How to get old versions of macOS. Visit that link and any within it using the Safari browser.

If you get a corrupted message...

Naw, not damaged, it's just that you have an old copy already in Applications, there are 2 cures...

Disconnect from the Internet & set the Date back to 2014, then you can run the Install MacsOS High Sierra App in Applications, so expired Certificates are not expired.

Or... trash the Install MacsOS High Sierra.app in Applications then a new copy more recent version can be downloaded.