Switching slot after firmware flashing failure

Hi. I have and issue while upgrading my Balance 310 from 5.x firmware to 6.3.3.
During flashing process I have power failed down. After that Balance 310 can not start, it has Power (green) and Status (red) are turned on all the time. I have tryed to reset it, but all in vain.
As far as I know, it has two firmware slots and second (backup) slot is made specially for such troubles, that I have.

Could someone kindly explain, how to make second slot active and boot the router?

This device is faulty. Please open ticket to proceed RMA.

Item is already out of warranty. Is it any options to fix it myself?

We don’t support this. We can offer the Out of Warranty Repair.