I finished the cube but got this on 1 side:
How do I fix it without ruining it?
It is fixable, I did it once but I don't know how I did it.
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Sign up to join this communityHold the cube so that the centre to be twisted is in the U face.
Then do the 12-move sequence:
R L U2 R' L' U
R L U2 R' L' U
What the first 5 moves do is to swap two opposite corners and two opposite edge of the U face. After a U quarter turn the moves are repeated to swap the other corner pair and edge pair. This means all the U layer pieces have been rotated 180 degrees around relative to the centre. When the U layer is then turned to its correct location it is only the centre that has twisted 180 degrees.
R U R' U (5x)
. Jaap's algorithm seems to be shorter though, and actually has reasoning why it works. $\endgroup$