How to Factory Reset BT510 loaded with legacy firmware?

There are two methods of doing BT510 Factory Reset.

Method 1: If the button is pressed for a duration exceeding 10 seconds and then released, the sensor will initiate a factory reset. This factory reset operation wipes out all stored logs, clears pairing information, and restores the configuration to its original settings. It takes approximately two seconds for the BT510 to reboot and become operational again after the reset, during which time it won't be able to detect button presses.

Method 2: You can do Factory Reset via nRF Connect for Mobile. This process is briefly explained in the User Guide in the Section 4.4, Example 6. Instead of the keyword “Reboot” you can use “Factory Reset.”
 
This is the JSON command that you can use: {"jsonrpc": "2.0", "method": "factoryReset", "id": 4}