Assuming you have the music installed in iTunes on the laptop, I suggest making a playlist especially for the phone (called iPhone perhaps!) Drag the music you want on the phone from the iTunes library of the laptop into this playlist, then, in the choices for what music to sync, deselect "all music" and opt for just syncing that one playlist. Click Sync.
When you want a change of music, drag out of the playlist songs you no longer want on the phone and replace with new ones then sync again.
That's how I've always done it. I don't keep much music on my phone and I change it from time to time so dragging old stuff out of the playlist and new stuff in before syncing seems easiest for me. I even connect the phone the the Mac using a good old-fashioned cable - none of this new wireless stuff!