1. Select the downward arrow below Edit Queries, then choose Edit Parameters
2. In the box that appears, delete the current 'authKey' (making sure to delete all of it, it's longer than the displayed box)
3. Paste in your new token (see here on how to get your new token) and select OK
4. Select Refresh and your Report will update using your new token. Tokens are valid for 3 months, after which you must generate a new one.
If you're using PowerBI Service then you'll need to update the token as above in PowerBI Desktop, and then publish to Service.