These steps are for MT5 on desktop. Menus differ in the web and mobile versions.
In the Trade tab, double-click the position, or right-click it and choose Close Position.
To close only part of it, enter the volume you want to close.
Click Close.
The closed trade moves to the History tab.
If you hold a buy and a sell position on the same instrument, you can close one against the other. In the close window, choose Close By in the Type field, then pick the opposite position. You pay the spread once instead of twice.