I have 2xRB750
Managed to upgrade one RouterBOARD (& then RouterOS) fine this morning.
The other is refusing to upgrade.
The new RouterBOARD goes on but then after the reboot it reverts back to the old version again.
Current RBOARD version is 2.36
Trying to upgrade to the current latest version, downloaded from the RouterBOARD website today.
Edit: Current RouterOS version is 5.4