Since official manufacturer links are dead, your best bets are enthusiast archives:
Android 5.1 brought significant improvements to the RK3188 platform compared to the aging 4.4 KitKat: rk3188 android 5.1 firmware
Flashing a firmware for a TV box (like an MK808) onto a tablet or a different model of TV box will likely brick the device. Since official manufacturer links are dead, your best
However, the "story" often ended with a bittersweet twist. While 5.1 brought new life, it also pushed the aging hardware to its limit. Some devices ran hotter than ever, and others lost their Bluetooth or Wi-Fi capabilities in the transition—a classic trade-off in the world of custom firmware. Since official manufacturer links are dead
: Generally faster UI response and more reliable background process handling.