You can choose any valid Mi Band firmware file or any APK file of official Mi Fit app.
Search your custom firmware on Mi Band forums or on google 'Mi Fit apk' for more details
For example, you can download official Mi Fit app APK from this website
http://apkmirror.com/apk/xiaomi-technology/mi-fit/
Then choose the downloaded file to start the firmware upgrade
Please ensure you have grant storage access permission to Notify app, and put the apk/firmware file locally on your phone internal storage (suggested folder /miband/mibandnotify)