Git: How to merge/update parts of a branch from/to main?

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; 开发者_如何转开发git merge machine/master?

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

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).