Download
the zip of the main branch. Unzip it into the lib
directory of your PIO project.
The directory tree should look something like this:
You may need to reinitialise your project to make your IDE notice the new library. (i.e. run
pio init
with the correct arguments for your project.)
See Getting Code out of GitHub if you want to install a different branch. (You wouldn't normally do this unless you're trying out a bug fix or pre-release version.)