To do a migration there are two options (A or B) and, depending on your Pumukit version can be also necessary to execute another additional migration steps after the the general migration procedure (A or B) (you can see this steps in the Specific migration guides down below)
A) Installation using git
In this case you only have to do a 'git checkout' to the desired tag.
For example, if you want to migrate from 3.4 to 3.9 you must stop pumukit, do git checkout 3.9.x in the folder and then launch pumukit again
B) Installation using composer.json
In this case you must change the selected branch in the file and after execute "composer update" to update