I tried to update firmware with the phone, it prompts memory full and suggest to connect with PC Garmin Express.
Connected with PC, It still cannot update because out of memory.
I tried Delete All Activities, Hard Reset, it still left 1MB available.
I use Windows 10 PC format the watch in FAT mode (FAT12) same with the watch original format.
Connect Garmin Express again, it restore some system folder in the watch.
Remaining 4-5MB, finally updated to v16.00 from V14 successfully.