Reducing the volume of an active investment effectively means selling shares or fractions thereof.
Let's suppose you bought 1.99 shares in nVidia when the share price was $227.55. After some time, this company's share price rose to $282.62 and you decided to take some of your profit. For this, you need to go to the "Active investments" section, open the active investment window and hit the "Close" button.
A window will then appear in which you must specify the volume of shares you would like to sell, then hit the "Close" button.
Shares may be sold, provided that you own the asset in question. You can select the volume for sale in the close investment window.
Be aware that when you reduce your volume (selling shares/fractions thereof), the opening price will not be recalculated.