Hello and welcome to Apple Support Communities, Klara.talidai.
If I understand your post correctly, you'd like to delete music that you've synced to your iPhone. I'd like to help.
These articles will help you delete items:
From your iPhone: Delete music, movies, and TV shows from your iPhone, iPad, or iPod touch
From iTunes on your Mac: iTunes for Mac: Delete songs, playlists, or other items
From iTunes on a Windows computer: iTunes 11 for Windows: Delete songs, playlists, or other items
Let us know if that resolves this issue for you.
Thanks for using the Apple Support Communities. Take care.