When viewing songs in your set, tap on the Edit button in the upper right corner to enter editing mode. When enabled, you have the option of:
Songs are added to your set in the order that they are pulled. Go into editing mode to rearrange them in the order you will perform. Use the reordering handles on the right to tap and drag the song into the proper position.
Note: If you are not using custom sorting when you reorder rows, you will be prompted if you would like to enter custom ordering.
While you you can remove songs from the set by swiping over each row from right to left and tapping on Remove button, you can do the same while in editing mode by tapping on the red minus circle on the left. Songs are removed from the set but are not deleted from your master library.
Tapping on the name of a song will open the Action Menu, allowing you to perform advanced actions on songs in relation to the set. This can be used to duplicate songs in a set.