It tooks me 2 hours to finish and in the end I have a not working iPhone without even the chance to close it:

1) during the opening I accidentally damaged one side of the metal structure attached to the screen and it doesn't come again inside.

2) Removing the old battery was REALLY hard but in the end I did it, on the other side I tried to switch the iphone on still opened and it worked, at the beginning, but after few hours it doesn't work and charge anymore...

Tomorrow I'm bringing it to an authorized assistance.