Well, i give it a try…

I am by no means a HU specialist. Mostly i spew it of at the FT before HU so i dont have to worry about that too much

First, if you feel unconfident playing HU i would suggest you put some volume (2-4hours/week) into HU cash game and get a feeling for how villains play.

If villains are adapting then we need to adapt to their actions. We always want to be one step ahead of our opponents. So how can we adapt if they start to play back at us.

1.) the easiest is we tighten up and start to fold more btns so when we raise and flat their 3bets we still have their range crushed. Also limping the btn is an option when stacks are short that they can easily reshove. Look at Mersons play during WSOP ME 3handed. He was Limping a lot of BTNs with almost his entire range to take Balsigers strongest weapon away. The all in reshove with 20-30BB. the same we can do HU. Because we are IP postflop.

2.) we start to 4bet lighter to put the pressure back on them. this obv works only if stacks are deep or short to push all in over their 3bet. This will of course lead to a higher variance in your game. so i would do this only if you have a very good understanding of  what ranges are good doing so.

using flopzilla and playing around with ranges should give you a feeling with which ranges you should r/f, r/4bet and r/flat in different spots. Its absolutely crucial that we adjust our ranges from 4-3handed to heads up. There is a nice article in the november issue of Bluff where Greg Merson talks about the mistake of having a wrong 3bet range for example HU. He is particularly discussing about the last hand of the WSOP ME.

hope this helps a bit

cheers