How to see the changes in a Git commit?

To see the diff for a particular COMMIT hash, where COMMIT is the hash of the commit:

git diff COMMIT~ COMMIT will show you the difference between that COMMIT's ancestor and the COMMIT. See the man pages for git diff for details about the command and gitrevisions about the ~ notation and its friends.

Alternatively, git show COMMIT will do something very similar. (The commit's data, including its diff - but not for merge commits.) See the git show manpage.

As mentioned in "Shorthand for diff of git commit with its parent?", you can also use git diff with:

git diff COMMIT^!

or

git diff-tree -p COMMIT

With git show, you would need (in order to focus on diff alone) to do:

git show --color --pretty=format:%b $COMMIT

The COMMIT parameter is a commit-ish:

A commit object or an object that can be recursively dereferenced to a commit object. The following are all commit-ishes: a commit object, a tag object that points to a commit object, a tag object that points to a tag object that points to a commit object, etc.

See gitrevision "SPECIFYING REVISIONS" to reference a commit-ish.
See also "What does tree-ish mean in Git?".

You can also try this easy way:

git show <COMMIT>