How to fall back PX4 bootloader from px4_fmu-v3 to px4_fmu-v2?

Hi all,
I connected pixhawk2.4.8( with the Firmware of PX4 1.9.1, px4_fmu-v3_default) to QGC through USB port on my computer.
But QGC says “Error on link Serial Device on ttyACM0. Error connecting: Could not create port. Permission error while locking the device”.
Do anyone can fix this issue?
Or just fallback to px4_fmu-v2_default?
How to download a fmu-v2 bootloader to a pixhawk2.4.8( with the Firmware of PX4 1.9.1, px4_fmu-v3_default)?

Thanks&Best Regards!