A garrison unit can exit a building in their Remaining Moves sub-phase, though not in the same turn that it garrisoned the structure. Place the unit in any desired legal formation, with at least one model of the rear rank 1" away from the building and no model within 1" of the building. Note that no model may be placed more than double their movement value away from the building. The unit may not move any further in a turn that it exits a building – it's too busy reordering its battle formation. This is the only way a garrison can voluntarily leave a building – it cannot charge out of a building, for example.
A unit that is Frenzied, or is otherwise forced to charge, still cannot do so if it is in a building – it must instead exit the building as close as possible to the enemy instead of charging.