I am unable to upgrade my Edge Explore from 4.30 to 4.60. The device performs upgrade validation (0-100%), turns the device off and does not turn it on. When I do it manually after some time the firmware is still 4.30.
You might try to install the update through the menu: System / About / Software Update
If no update available remove the file GUPDATE.GSC from the folder ..\Garmin and let the Explore sync again by Garmin Express (latest Windows version is 188.8.131.52
If this does not resolve your issue, then do a full factory reset of the device (all settings are lost!!) and try again using Garmin Express.
If you are using Apple I don't know if there are known issues causing troubles with updating firmware.
In my case the update worked without issues (Windows 10 64bit, GE 184.108.40.206