In one of the Tanpura tabs, scroll to the tanpura you want to customize and click on the EDIT button. You will then see the Strings editing pane appear.
There you can scroll through the strings and adjust the Gain and/or Trim sliders the way you want. You can keep the tanpura playing during that step so you can hear the changes in real-time.
If you are not sure about your modifications, just click on the RESET button and everything will go back to default for that Tanpura.
Changes are stored globally per tanpura, so they will be heard whatever the Tanpura tab and session you select.