Toward the bottom of each problem set is a section entitled How to Submit. Follow the instructions therein to submit your problem sets, and within a few minutes, you should see your scores at cs50.me/cs50x! You may re-submit work as many times as you'd like; we will record your most recent submission's score.
If you started CS50x before 2021, know that scores from CS50x 2020 (but not before!) will be imported into cs50.me/cs50x within the first 2 weeks of January.