How do I edit my iPhone playlists through iTunes?

Hi guys, so I upgraded to the new iTunes 2 days ago. My phone has a bunch of horrible music on (I wedding DJ) which I want to remove. I usually make a playlist in iTunes, then select & sync to put it on my phone. But now, for some reason, the only way I can alter the 'music' playlist (to go on my phone (seen here in blue)) is by adding songs. It won't let me delete the crappy songs on it already! I have tried EVERYTHING! Right click, delete, drag & drop, NOTHING! How do I get this ****** music off my phone and replace with playlists I want? HELP!!! PS If I click edit playlist (which takes me to this window then it shows my ENTIRE library to select from. Is iTunes suggesting I have to deselect the songs from a playlist of 10 000+ songs?! Is that the solution?! If I delete all the music from my phone, this crappy playlist just gets sync straight back on as soon as I plug it into my computer. HELP!!! Thanks in advance 🙂