Just got a software update to version 5.10 through Garmin Express. No mention yet on the Forum or elsewhere, so the usual (...) question:
What or where are the Changenotes?
If you can still connect the device to the PC in mass storage mode you can try to - if present - delete the file GUPDATE.GSC from the folder ..\Garmin (file may be corrupt)
Also check if the folders CustomMaps, Newfiles, RemoteSW and TempFIT are empty. If not, first save the contents as a backup copy and make the mentioned folders empty. Then detach the device from USB safely (Windows System Container), power OFF / ON, reconnect the device through USB and see if Garmin Express succeeds now in the update. A fresh update file GUPDATE.GSC should be copied to the folder ..\Newfiles during the Synch phase. Wait for the green banner that Synch has finished successfully. Detach the device again from the PC, power off / on and the device should indicate that an update is ready for install - confirm for installation. If nothing shows up, check Menu / System / About / Software Update.
Also respect @mattsches hints above