If you placed a tile by mistake, before deploying a meeple you can undo tile placement:
1. using menu Game --> Undo
or
2. Ctrl+Z.
Lo mas importante de saber es que si ya has colocado tu seguidor, no puedes cambiar nada. Pero como dijo SeñorNúmeros, puedes cambiar la loseta antes de que colocas un seguidor.
The most important thing to know is that after you've placed your follower, you can't change anything. But like MrNumbers said, you can change the tile placement before you place a follower.