Changing the volume or position size is only possible for orders about to be placed, and not possible for already placed orders.

The volume can either be inputted manually at the time of placing an order, or by using the arrows for decreasing and increasing. 

What is the volume of a position?

The volume of a position relates to the total number of shares or contracts exchanged between buyers and sellers of a securities. The number of lots, as well as the kind and size of lot you purchase or sell in a trade, define the size of your position.

In trading:

• 1,000 units = A micro
• 10,000 units = A mini
• 100,000 units = A standard 

How to increase or decrease the volume of a position in MT5

Unfortunately, it is only possible to edit volume or lot size before finalizing an order. To change the volume value, manually input your calculated position size or use the arrow up or arrow down button.

It is impossible to edit the volume of an existing position/trade or order in MetaTrader 5 as you will have already entered the trade. If you want to change the volume, you will have to close your open trade and enter into a new trade with your desired volume.