Hello,
To make an attended transfer, while you are in a call, tap the 3 dots menu on the top right and select "Add call". A new screen will appear. You can type the second number in the search field or look for one in your list or press the dialpad and dial the second number manually.
The first call will be automatically put on hold. After you are finished talking on the second line, press the 3 dots again and select "Transfer call".
Unattended transfer is similar but instead adding the second call with "Add call", directly click on "Transfer call" and type the number you would like to transfer the call to.