How to add more songs on iPhone without deleting old ones?

When I plug my iPhone in to add more songs to my phone it always says that I need to remove and sync my phone. When I do this it erases everything on my phone and only adds the new songs I want to put on my phone. How do I add the songs without deleting what's previously on there?

You have to check the box next to what you already have In there say like if you havdee song "A" and want song "B" you'd have to check song "B" then sync… This would erase song "A"… So check whatever songs you want to keep before syncing

1.Connect your iPhone to your PC/Laptop.
2.Open iTunes
3.Click on your iPhone's name on the sidebar
4.Click on music on the top
5.Enable "Sync Music"
6.Enable "Entire Music Library"

Hope this helped