As of May 12th 2018, not possible. A bad design choice, going with one-click registration which is available during the contest, but cannot be undone and prevents the user from registering to another contest.

I think people here on Codeforces often don't appreciate the amount of great work the developer team here has done. Making a functional contest system & community is kind of hard already; making it an enjoyable experience is, as seen from numerous alternatives that sometimes fail to accomplish that, an achievement that requires both sofware engineering ability and great vision.