When you are using spotify on your mobile without premium you cant really change song, you can only shuffle your lists and have the ability to skip 5 songs or whatever. I found a way to choose the song you want without having to "shuffle"

Just start a song and press "spotify connection available", then if your pc is on the list choose that, when you have choosen your pc, you can listen to whatever song you would like but it plays on your pc, however if you press "spotify connection available" again, you can switch back to your mobile and the song you choose will continue.

I dont know if this works in your car or if your away from your house. I was just taking a dump and saw that I could choose from my pc and the change back to the phone.