How to update macOS version 10.11.1 to 11.5 ?

A MacBook 6,1 cannot be updated beyond macOS 10.13.6. See the following from Identify your MacBook model - Apple Support

MacBook (13-inch, Late 2009)

Model Identifier: MacBook6,1

Part Numbers: MC207xx/A

Newest compatible operating system: macOS High Sierra 10.13.6

To upgrade, be sure to have a backup and see instructions in: How to download and install macOS - Apple Support

If you update from El Capitan to High Sierra, you may want to install FireFox 115 ESR. It doesn't get new features any more, and Mozilla won't provide critical security updates for it after September 2024. Still, it may get critical security updates for a few months. El Capitan's version of Firefox stopped getting them back in late 2021.

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