You can close a position in multiple ways. On the chart itself, by pressing the X next to a position is one way.
In the “Open Positions” section at the bottom, you can see the X on the right hand side of each position which will close it if pressed.
To close multiple positions at once, press the “SELECT” button in the top right side of the “Open Positions” section. By pressing close, you are able to close all open positions at once, however by pressing the downwards arrow beside it you can filter on which positions to close based on if there are buys, sells, profitable, or losing. You can also tick positions individually to close a number of specific ones.