Reducing the volume of an active investment in the Portfolio account is essentially the sale of storks or part of the stocks.
This can be useful for partially locking in profit by selling part of the available stocks (including fractional shares of one stock).
How to sell part of an active investment on a desktop?
Go to the "Active investments" section in the Portfolio account.
Select the active investment and click "Sell".
Specify the volume of shares to sell and click "Sell".