How to upgrade my 2010 iMac 27 in?

Sorry, no, High Sierra 10.13.6 is the latest possible.

Also see the the following from Identify your iMac model - Apple Support

iMac (27-inch, Mid 2010)

Model Identifier: iMac11,3

Part Number: MC510xx/A, MC511xx/A

Newest compatible operating system: macOS High Sierra

You can't update the version of macOS on that machine any further.

If you don't already have Firefox, you might want to consider downloading and installing Firefox 115 ESR. It's not getting any feature updates, but Mozilla says they will provide critical security updates until September 2024.

https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

Please also remember a 2010 is very obsolete and is living on borrowed time, have you considered upgrading?