I would be very careful in your situation. Just changing your mind by no reason might make you look wierd or non reliable. Imagine buying a TV, when you ask for the price, its €500, you are happy. Then you reach for your VISA card, but then you are asked for €550 instead - but it's still the same TV. Would that feel nice? Would you trust that salesman and buy more from him? Probably not. The same goes for you and your future boss. He might have some understanding if this is your first negotiation though.
What you can do, is to try to figure out a great reason why you have increased your salary demand. I'm not sure what applies to you (don't lie!!), but some reasons might be:
"I just read the details in the agreement, and understand overtime is not paied out in this contract, which I expected. I would like to compensate my initial salary request for that by xxxx"
"I was just called by another big company and was offered a higher salary over there. I still would like to work with you, since Blah Blah, so if you could match that offer, I'd be happy to join".
or, perhaps a bit vauge but probably the easiest "excuse" to request more salary. That is, say you misunderstood the position and that kind of position should pay more.
"I had the impression that this was a very basic entry level position, but during our last discussion I start to realise there is quite a bit of responsibility involved from day one. A reasonable salary for that kind of responsibility would rather be XXX than what I previously stated".