In order to reduce your position (holding) or sell part of it please follow these easy steps:
- In the list of stocks/positions, go to the "Gains & Losses" tab,
- Find the stock that you want to partially sell a holding of, tap the ( i ) icon next to its symbol,
- Then tap on the ( i ) button next to the holding you wish to reduce (partially sell,) 
- Change any parameter of the current holding. For example, the number of
shares. You may also correct the price per share, change the date of
trade, add note or specify a commission for your transaction.

Alternatively, you can keep your SOLD transactions in a separate "Sold" portfolio -
e.g. the one which is located in your list of portfolios by default.