RosaAlba wrote:
The short answer is that you can't.
To the OP: macOS 10.13 will work here, and is available for download. It’ll fix the immediate problems with digital certificates you’re probably having with Safari and other tools, though High Sierra is no longer supported, nor is High Sierra receiving patches from Apple.
There are no updates showing for your Mac because it's too old to run the latest version of MacOS.
To the OP: Apple differentiates updates—what the OP has undoubtedly been checking—from upgrades. macOS updates for macOS are minor changes, and are unlikely to perturb apps, a d are easy to get. macOS upgrades can potentially break apps, and upgrades must be specifically requested.
An early 2011 MacBook Pro cannot be updated past High Sierra (10.13) and, as that's now 4 versions old, it's not available on software update.
To the OP: Here is how to download many of the older macOS versions directly from Apple, including how to acquire and install High Sierra ➡️ How to get old versions of macOS - Apple Support
To the OP: Have a complete and current backup prior to installing High Sierra.
To the OP: Do expect to be replacing this MacBook Pro too, as it’s a decade old and well off of Apple software, hardware, and repair support.