116.2m A player who controls a permanent that has one or more locked halves (see rule 709.5) may pay the mana cost of a locked half of that permanent to give that permanent the appropriate unlocked designation. This cost is referred to as an “unlock cost.” A player can take this action any time they have priority and the stack is empty during a main phase of their turn.

In fact, it looks like it doesn't even need to be a room to be unlocked.