You can completely rearrange your unit, so that it is facing in any direction, by giving up all other movement and shooting.
A unit of troops can change the direction in which it is facing and rearrange its formation all at once by means of a maneuver called a reform. The leader issues the order to adopt a new formation and the troops move to assume their new positions.
Keeping the center point of the unit the same, arrange the unit into a new formation of as many ranks as you please, facing whichever direction you wish. Remember that none of the models in the unit can move more than twice their Movement rate.
A reform is a complicated manoeuvre and ordinarily prevents the unit from moving any further that turn, and also stops it from firing missile weapons in the Shooting phase. However, it's worth noting that if the unit has a musician, there is a chance it will be able to carry out the reform and still be able to move, and even shoot if it has the appropriate weapons (seefor more details).
When a unit reforms, it can change its number of ranks or files (A), the direction it is facing (B), or both (C).