In the Teams app left hand sidebar select an icon you want to move, and click and drag it to the place you want it.
This is useful if say the Teams Icon has been at the second from top for the last 2 years and then suddenly Microsoft move it,
for some reason.

If you want to remove one of the icons completely right click it (CTRL click) and select Unpin. This can be useful to stop accidentally selecting things you never use like “Assignments”.