Of course, it is possible to watch already started programs from the beginning. Press the Guide button while a program is running, and the program is automatically highlighted. Press OK once and you will see the "Details" of the program. Press OK again, then you will be asked if you want to watch "live" or the current program from "beginning". Now select the desired option by pressing the arrow key. Press OK to finish the process.
For programs that are completely in the past: also open the EPG and select the desired program (if necessary, you can select the corresponding day with the green button). Press the OK button once, the "Details" of the program will appear. In this view you have the choice between "Watch now" or "Record" in the upper right corner. Press OK to select "Watch now" and the program will be played from the beginning.
You can also use the blue REW button to find the beginning of a program. But using the GUIDE is faster.