What is the JSON string to change the TX power in the BT510 outside of the BT510 app?

The following string should be sent over VSP, where 8 is 8dBm

{"jsonrpc": "2.0", "method": "set", "params": {"txPower": 8}, "id": 1}
Valid values include:
-40 dBm, -20 dBm, -16 dBm, -12 dBm,  -8 dBm, -4 dBm, 0 dBm, +2 dBm, +3 dBm,  +4 dBm, +5 dBm, +6 dBm, +7 dBm, +8 dBm

See the user guide for further details.