1) Check the pKa of the residues in the PROPKA server. Then, protonate or deprotonate residues according to the pKa and given pH. You have to use these protonation states in the simulation.
PROPKA: http://propka.ki.ku.dk/
2) Constant pH MD simulations: It is computationally expensive as it based on the lambda-dynamics. For, further details see the following paper.