During an open game state in the Main Phase, the turn player can manually change the battle position of any individual monster they control. Each monster's battle position can only be changed manually once per turn.
If a monster is Summoned, Normal Set, or declares an attack (even if the attack was negated or cancelled due to a replay), it cannot manually change its battle position for the rest of that turn. If a monster battles due to a card effect that conducts damage calculation without declaring an attack, it can still change its battle position afterwards.
You cannot change monsters to face-down Attack Position, neither manually nor by a card effect. Face-down Defense Position monsters are always flipped when changed to Attack Position. If a monster is manually changed from face-down Defense Position to face-up Attack Position, it is Flip Summoned (which is both a Summon and a battle position change).
Card effects that prevent monsters from changing battle positions only prevent them from changing their battle positions manually (except "Raging Cloudian")—their battle positions can still be changed by card effects.