Traditional training would tell you need to point into wind to put in or take out a reef, but the truth is all you NEED to do is take the pressure off the Luff of the sail to allow it to travel up the mast or down.
You see, as a sail powers up it creates pressure which pushed the sail forward onto the mast. If your sail has battens this will mean that there is a huge amount of pressure pushing the batten cars against the mast and making them NOT want to slip up and down without a fight. This fight can damage things.
So traditionally you must turn up into wind, take the wind out of the sail and pressure off the mast cars so you can lower or raise without ripping things.
But you can also take the pressure off in other ways. If your headed downwind and you’re able to pull the boom into the wind so that the sail starts to unload and flap, from behind, this will allow you to release your reef, or put one in.
Likewise if you have very changeable winds and the pressure is coming on and off the sail you can carefully use the lulls to move the main halyard.
Take care, this needs to be monitored carefully so you don’t damage things and probably best using a manual winch handle but you can apply pressure onto the halyard, or off and wait for a lull to allow it to move and them repeat.
NOTE: this is NOT a fast manoeuvre but a carefully controlled slow and easy action.