please help me :)

hai ok so im new at this whole thing but ive had my iPhone 6 for several months now and ive never really used iTunes on any type of laptop before. so I have two questions today actually...

Question 1: I first did this with my best friends computer and so my iPhone is now synced with hers. is there any way to un sync my phone from her laptop without having to have the laptop present?

Question 2: Whenever I try and just choose a song and I click "Add To" it pops up with " iPhone cannot sync due to error -54" how do I fix this?