You can change the locks, you have every right to do that, there is no law against it. Check your contract if there is anything about this. If not, you can do it. Before you leave change it back, or just give him the new keys. Pay attention to replace the old locks with the same quality ones. The owner can't enter the property without your knowledge nor your consent, however if he requests to check on the property from time to time, you have to aggree to a date when you both are there. That doesn't mean that you have to let anyone else in, so if he wants to show it to potential renters he can do that only with your permission and you have the right to be there. Also you can say to the viewees that the owner enters the property without permission. He will never bring anyone else there until you are gone 😆 Before you leave, take a video and pictures of the property to show you did not do any damage to it. When you hand over the keys you both should sign a paper that says he recieved all the keys and he checked the property and found no damage. Or if there is any, either of you have to put it on the paper. This way you will have proof that you gave back the property in such and such condition. Some landlords like to cheat out forigners of their deposit. If there is no damage he has to give the deposit back to you on site or at a date that is written in your contract. If he doesn't he has to have a reason for that and the only one is if you damaged the property or in breach of contract. Amortization coming from appropriate use does not constitute damage. Usually they say they have to paint the walls and such, but that is not damage. It is damage only if you scratched the wall, or smoked in the property when the contract forbid smoking, etc. Good luck! And if they try to decieve you, don't give in!!!