I forked someone's repository on GitHub and would like to update my version with commits and updates made in the original repository. These were made after I forked my copy.

How can I pull in the changes that were made in the origin and incorporate them into my repository?

You have to add the original repository (the one you forked) as a remote.

From the GitHub documentation on forking a repository:

Once the clone is complete your repo will have a remote named “origin” that points to your fork on GitHub.
Don’t let the name confuse you, this does not point to the original repo you forked from. To help you keep track of that repo we will add another remote named “upstream”:

$ cd PROJECT_NAME
$ git remote add upstream https://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git
$ git fetch upstream
then: (like “git pull” which is fetch + merge)
$ git merge upstream/master master
or, better, replay your local work on top of the fetched branch
like a “git pull –rebase”
$ git rebase upstream/master

There's also a command-line tool (gh) which can facilitate the operations above.

Here's a visual of how it works:

See also "Are Git forks actually Git clones?".