On my S750.52 it's 1.2S4F3 and the EPG Software is 3.2.11. Don't know whether the slight difference in model number is important for the OS version - I suspect not.
First of all don't try an update if you haven't got a strong signal - you could corrupt the EEPROMs.
To update: unplug from the mains, wait a moment and then plug back in while keeping the backup button on the STB (NOT the remote) pressed in. When all the lights on the front come on let go and just wait until they go off again - this could take several mintes.
If you have the STB connected by scart you can follow the progress on the TV