Configure default upstream remote branch
The first time you pull changes from a repository (or if cloning via Magit), the default upstream remote and branch is set.
F u will pull all the changes from the default upstream remote branch.
Changing the default remote
To change the default upstream, first unset the default value and then add a new default.
F to open the pull menu
C to open the configuration of the local repository
u to unset the default upstream branch (removed the description too)
u to add a new upstream branch as the default, showing a list of all possible remote branches.
RTN to choose a branch
G-g to close the configuration menu. Then either
u to pull from the new default upstream branch, or
C-g to exit the pull menu.