How do I reset 'master' to 'origin/master'?

Git supports this command:

git checkout -B master origin/master

Check out the origin/master branch and then reset master branch there.

As KindDragon's answer mentions, you can recreate master directly at origin/master with:

git checkout -B master origin/master

The git checkout man page mentions:

If -B is given, <new_branch> is created if it doesn’t exist; otherwise, it is reset. This is the transactional equivalent of

$ git branch -f <branch> [<start point>]
$ git checkout <branch>

Since Git 2.23+ (August 2019), since git checkout is too confusing, the new (still experimental) command is git switch:

git switch -C master origin/master

That is:

-C <new-branch>
--force-create <new-branch>

Similar to --create except that if <new-branch> already exists, it will be reset to <start-point>.
This is a convenient shortcut for:

$ git branch -f <new-branch>
$ git switch <new-branch>

Originally suggested:

Something like:

$ git checkout master

# remember where the master was referencing to
$ git branch previous_master

# Reset master back to origin/master
$ git reset --hard origin/master

with step 2 being optional.

I think even VonC's answer has complexity compared to this option:

git update-ref refs/heads/master origin/master
git reset --hard master

git automatically logs every value of a ref (through the reflog). So after you run that command, then master@{1} refers to the previous value of master.

VonC's answer is correct, but it wastes time checkout out the old value of master into the filesystem.

If you care about orphaned objects in the repo, then you can run git gc

If you are already on master you can do the following:

git reset --hard origin/master

It will point the local master branch to the remote origin/master and discard any modifications in the working dir.