The Fighter feat Dual-handed Assault (CRB p146) lets you grip your weapon with 2 hands, strike, and release it with one of your hands, all in one action.
How many actions would it be without the feat?
According to Archives of Nethys: Dual-Handed Assault.
You snap your free hand over to grip your weapon just long enough to add momentum and deliver a more powerful blow to your opponent. Make a Strike with the required weapon. You quickly switch your grip during the Strike in order to make the attack with two hands. If the weapon doesn’t normally have the two-hand trait, increase its weapon damage die by one step for this attack. (Rules on increasing die size appear on page 279.) If the weapon has the two-hand trait, you gain the benefit of that trait and a circumstance bonus to damage equal to the weapon’s number of damage dice. When the Strike is complete, you resume gripping the weapon with only one hand. This action doesn’t end any stance or fighter feat effect that requires you to have one hand free.
Changing your grip on a weapon with the two-handed trait (not to be confused with actual two-handed weapons) like a bastards sword requires an interact action, as noted at Archive of Nethys: Wielding Items.
Comparing this to the same activity without the feat:
Total: 2 actions
With the feat:
Total: 1 actions