Firmware Downgrade

Hi there. 

I think there were some topics how to install previous firmware version on Edge, but I could not find one.

I have Edge 520, FW 13.00. 

1. I would like to downgrade firmware to earlier versions (something around 7.xx or something like that) - is that possible and how to do it. 

2. Is there any limit in version number (I bought it with some very old FW (2.5  I think) and it used to turn on instantly - would love to have that again)

3. Which version is most battery-friendly and stable

Thank you in advance.