I can't find this in the rules. If someone is wielding a weapon with the 2-handed property (it has a 1h requirement, but does a larger damage die when wielded with 2 hands, for example a bastard sword or a dwarven waraxe), do they need to use an interact action to change their grip from two handed to one handed, or from one handed to two handed? The release action is free, so one might think that changing from two handed to one handed would just require a release action.
Releasing a single hand from a weapon is covered under the "release" free action. Putting a second hand on a weapon should be an interact action. I'm not sure if it's explicitly stated in the rules, but I'm pretty sure the devs have confirmed it.
It's covered explicitly in the Equipment Chapter. Check out Table 6-2 on p. 273.
As stated, it normally is a free action to go from two-handed to one-handed and an interact action to go from one-handed to two handed.
However, there is also the fighter feat Dual-Handed Assault, which allows a character to switch from one-handed to two-handed, make a strike with the weapon, and switch back to one-handed with a single action.