how to update macOS High Sierra 10.13.6 to the current macOS version?

Is this for the 13-inch Macbook Air in your equipment profile? If not, what MAc.

Your upgrade path solely depends on the sub-model of your Mac, which we do not know, but you can quickly find by doing "About this Mac..."

To run any higher OS, your computer must be 2012 or newer. To run the CURRENT macOS version, your computer must be in this list:

Most 2011 model Macs were released with 10.6 installed.

Apple has now provided you with six or seven additional upgraded versions of MacOS at no additional charge. Your almost twelve year old Mac is no longer considered capable of running the latest MacOS, so no more upgrades are available.

It can still do everything its could do the day you took it out of the box, and a whole lot more. Your Mac has not shrunk, and no one has taken anything away.

If you need your computer to do more, it is your expectations that have expanded.

My Mac is a 2011 model - so the only way I can use a supported OS is to buy a new machine?