Decreasing the RSSI value of Puck beacon

You can use NRF.setTXPower(...) to set the actual transmit power of the Puck.

However I think what you're referring to with:

Some beacons have an app that we can manually configure the RSSI at distance 0

Is actually just a calibration number that gets transmitted in iBeacon advertising packets.

Check out https://www.espruino.com/iBeacon - if you transmit iBeacon packets then you can configure that value