Hi,
FW v1.0.9.42 is the latest stable version. You just have to install its files (Firmware or .fw file and Resources or .res or .bin file) using Notify and Fitness. What's the problem?
If you don't know how to install those files, follow step 3 at
THIS PAGE
(You also can install Mi Fit v4.0.17 and it will upgrade your Mi Band automatically)