Adding or removing teams/games/weeks mid-season
Unfortunately, there isn't currently a great way to add or remove teams or change the number of games or weeks in the middle of the season and modify the remaining schedule seamlessly.
You can either
manually modify / add / delete games on your schedule; or
copy your schedule (with the new teams box UNchecked – you want to use the same teams otherwise your standings won't work), export as CSV and then import those games to your existing schedule.
There isn't any way to reconcile the new schedule to your existing one, so you will in all likelihood see repeat matches between the same opponents in the newly generated weeks. You can try fixing those by hand, but this can be a very difficult process.
If you're unsure how to add or remove teams, see this article (BUT do not regenerate your whole schedule as indicated there as that will get rid of your already played games).
If you've only played one game
If you've only played one game so far, you could make a new schedule with the new total number of teams and rename or swap teams in such a way that Week 1 is exactly as you played it already.
This will ensure that the rest of the season flows from the games already played so you don't have any unnecessary repeats.
Erasing remaining weeks
If you're following the instructions above using the copy/export/import flow, you'll probably want to erase the existing games first.
You can delete everything after week n by changing your number of weeks under Settings to n, i.e. the number already played / want to keep. If you set your weeks to 2, for instance, it will ask you if you want to delete all games after week 2.
Then change it back to the actual number you'll play and import your games.