Can i force update bios in Venue 11 pro 7140?

After flashing A15 bios, my venue is too slow and takes too long to boot.

I tried to downgrade bios to A13 but i found out it is blocked in A15 bios.

But... why i cant even flash same A15 bios? it reboots after showing black screen for 2 seconds and nothing happened!

Please! its sooo uncomfortable that every time i turn on my venue i have to go into one-time boot menu and choose boot option(of cource theres just one boot option) and wait for 2 minutes to get windows work.

I know that best way is getting dell service but i want to do something before i deal with dell service.

Please... im so desperate..