Many models carry two or more combat weapons, giving them a choice of which to use upon entering the fray:
If a unit is equipped with two or more combat weapons, you must choose which it will use when its combat is chosen during Step 1.1 of the Choose & Fight Combat sub-phase.
The entire unit, including command, must use the same weapon (with the possible exception of units with the Motley Crew special rule). Characters can always choose separately – they are not obliged to use the same weapon as a unit they have joined.
If a unit champion has the option to be equipped differently to the rest of their unit, they can choose which weapon they will use separately.
Unless the weapon itself states otherwise (as do lances, for example), a unit cannot change weapons between rounds of combat if it remains locked in place and engaged in combat. The weapon chosen for the first round of a combat (i.e., the first round fought following a charge) must be used for the duration of the combat.