check the current branch

cd ~/MagicMirror
git branch

output example

* _fix_clipping
  develop
  master

and if the highlighted (*) name is NOT master then

rm package-lock.json 
rm package.json 
git checkout master

use git branch

to verify master is selected before continuing

then do the commands as listed in the documentation

if you are using my upgrade script, it will handle all this for you.