Parting may be such sweet sorrow, but when it's time to say goodbye, look no further.
It is important to remember that if you leave a board of which you are the only member, the board will be deleted, and the content can not be recovered. Leaving a board will not delete any content from it if other people have joined it, too. Your friends will still happily have their files (though we're sure they will miss you).
Mobile
Leaving a board is simple: all you have to do is select the board you'd like to leave, and then click the three vertical dots next to Share. As soon as you do this, a drop-down menu should pop up. Then, just click the Leave board option. This will remove you from collaborating on the board.
Web
Leaving a board from Collect web is just as easy. Open the board you want to say goodbye to, click the three dots in the top right, and then Leave board.