My friend gave me his mobile phone to help him to change the SIM card I dropped the phone accidentally and the screen cracked do I have to pay for the phone to be repaired

In general, we rule like the Ramban that if someone is asked by his friend to do a favor and help, and damage was caused, he doesn't have to pay anything unless he was really negligent. On the other hand Tosafot disagrees, so it's good to arrive at some kind of compromise payment. This is all very general and if both sides agree on the basic facts.