SHELL

copy from master to branch

#If you can use: git merge 
git checkout old_branch.
git checkout master . #With period
git add --all
git commit -m "Copy from master to old_branch"
git push -u origin old_branch

copy branch to master

git checkout master.
git checkout branch_from_which_you_have_to_copy_the_files_to_master . (with period)
git add --all.
git push -u origin master.
git commit -m "copy from branch to master"