I never found a direct way for this, but one method that lets one un-register is:
Go to the list of the registered participants, and find yourself in there (you can do this easily by switching over to the Friends list instead of the All one). Once there, you will see a red cancel icon next to your username. Click on it and that'll do the trick.

https://codeforces.me/contestRegistrants/1878/friends/true

click the × on the right of your handle

upd: sorry it's on the left, i'm too retared

upd: no no no it's on right.

Hey bro! I was really scard and worried of this too but acutally @MikeMirzayanov thought of this situation so it's OK!

If you will not make submissions during a contest your rating will be safe and it will not be changed! Not a need to un-register!!!

I hope this was helpful bro!.