I am looking into upgrading more than one site from 5.5.1.2 to current. Is there an upgrade path?
The best update you can do directly is 5.6.4.0. That would enable you to continue running the site in the latest web servers. php7.4 etc.
If you want to get to 9.2, then you need to build new sites. There is no upgrade path beyond 5.6.4.0.
You may be able to help that along by exporting content as cif.xml from the 5.6 site and importing to the 9.2 site. You will need developer skills or to get a developer to do this for you. That may work, or it may fail. Even if an an export/import works it will be incomplete and you will need to manually correct/adjust content and design.
The bottom line is, unless you have many 100s of pages, it is easier and more reliable to start a new site and spend a few days copy/pasting content the hard way.
Another thing to bear in mind is which addons/theme/application code you need to use. In many case you will need to look for an equivalent because your existing addons/theme/application code will not work on v9 and there is unlikely to be a direct v9 update.