Put yourself in the shoes of the person who has offered you the role. You would want to know if you were them and want to know why.If you went through an agent make sure they are passing on message in the way in which you would want and if possible try and talk to the person you are letting down. It's imperative that you try not to ruin the relationship as you may well cross paths in the future.It also depends why you've changed your mind but they may be able to fix this problem by offering you more money, more flexible working, more autonomy, more focus, more support etc...If they react badly then they probably weren't the right person to work for anyway.In my experience go with your gut. It's a brave thing to do to change your mind as long as you've done it for the right reasons and treat everyone professionally.Hope this helps.James