Your local repo keeps a list of "remotes," or other copies of the repository that it knows about. To list all the remotes you have and which URL they point to, use this command:
git remote -v
Look at the list and work out which remote(s) need to update to a new URL. The first column is the remote name, and the second is the URL where the repo exists (you will see them all twice, one labelled "fetch" and one labelled "push;" this allows us to have different read and write endpoints for the same remote name but in practice is vanishingly rarely used, so just try to ignore the duplication for the purposes of this maneuver).
First, delete the remote that points to the old location, so if the remote you want to change is called "upstream" then the command would be:
git remote remove upstream
Next, we need to create the new remote. Get the URL of the repo (copy it from the GitHub page, or whatever works for the hosting system you use) and create the new "upstream" repo like this:
git remote add upstream [paste url here]
Note: Git does not actually check that you've supplied information that will work at all! It might be worth following up with a
git fetch upstream to confirm that you can communicate with the repository as expected.
All these settings are in Git's
.git/config file in a fairly easy format, so you can also go there to make changes (especially handy if you need to update a lot of remotes for some reason). Hopefully, if you do come across this fairly unusual situation, the information here helps you to get back up and developing quickly.