Force my local master to be origin/master

To have origin/master the same as master:

git push -f origin master:master

Discussion on the parameters:

-f is the force flag. Normally, some checks are being applied before it's allowed to push to a branch. The -f flag turns off all checks.
origin is the name of the remote where to push (you could have several remotes in one repo)
master:master means: push my local branch master to the remote branch master. The general form is localbranch:remotebranch. Knowing this is especially handy when you want to delete a branch on the remote: in that case, you push an empty local branch to the remote, thus deleting it: git push origin :remote_branch_to_be_deleted

A more elaborate description of the parameters could be found with man git-push

Opposite direction: If you want to throw away all your changes on master and want to have it exactly the same as origin/master:

git checkout master
git reset --hard origin/master

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