How to add songs to your iphone from a new computer?

I got a new laptop, and I synced my iPhone to it. Now it stores my iPhone backups and my songs that are already in the phone is not deleted. But I noticed that songs from my new library(in my new laptop) isn't transferred to my iPhone. What do I do if I want to start adding songs from my new laptop to my iPhone? Detailed instructions will be appreciated, thanks in advance! X

Pela entrada usb