To change or edit a label on a board:
1. Press the three dots 
in the top right corner, and then "Labels"
2. This opens the Label Management window. Here you can rename them by typing straight into the text area, choose a colour by clicking the coloured dot and choosing a new colour, add a new label by hitting "Add a new label" at the bottom of the window or delete the label from the board by pressing the bin. Hit "Save" when you have finished making changes.
