Question

I’m a git newbie, and this is what I currently have:

master: A -- B -- C
          \
           \
branch:      X -- Y

1. How do I Update commit Y with
   changes from B and C?

   Is this simply: git fetch machine
master; git merge machine/master?

2. Push changes of certain files from Y to C?

Answer 1

For 1:

git rebase master branch

This will result in

master: A -- B -- C
                   \
                    \
branch:              X' -- Y'

For 2 (while master is checked out):

git cherry-pick -n Y

This will apply changes from Y to your working tree and you may amend C with the changes you want.

See git-rebase(1) and git-cherry-pick(1).