The BTS766 does not have user-flashable firmware. If a seller offers a "firmware.exe" file, it is likely malware. The "best" version of this speaker is simply the one that works out of the box. If your unit is glitchy, use the (Hold Power + Volume Up for 10 seconds) rather than trying to update it.
Below is the for the most likely candidate (a modern Bluetooth Speaker, using Bose standards as the benchmark for "best"), followed by troubleshooting for generic models. bts766 speaker manual best
The phrase implies you want the most readable, accurate, and helpful version of the manual. Unfortunately, because the BTS766 is sold by dozens of sellers on Amazon, AliExpress, and eBay, the included paper manual is often poorly translated. The BTS766 does not have user-flashable firmware