If you are wanting to watch something that had been on earlier in the afternoon then you can scroll back the same day on the EPG by clicking the Left arrow on the navigation wheel and look for the Start Over icon. (If you use the fast forward keys you scroll back 24 hours at a time so won't be able see programs from earlier the same day)
If the Start Over feature is available you will notice an icon which looks like a circular arrow pointing backwards, often with a "play" symbol inside, which indicates the ability to rewind and watch a live program from the beginning.
Also when you press the "OK" button on a program, you might see a "Start from beginning" option if "Start Over" is enabled for that show.