To switch to groups edition mode you just have to press 
in the top right corner of the Light Control view. The groups start to wiggle to show they are editable. You can now rename a group by touching its label or delete it by touching 
.
Be careful, deleting a group will modify the presets, sequences and dashboard's widgets that interact with it (the action is irreversible).
You can also reorder your groups: long-press a group for about a second until it lifts up, then drag it to its new position in the list and release it.
Do this for all the groups you want to rename, reorder or remove. When finished, press 
at the top right to save and cancel group edition mode.
Your group is now ready and allows you to change the color, dimmer, position and many other properties of your fixtures. Come and take a look at everything you can do in the group control panel.
Visit the documentation to control group's properties
