1. The video you cite is for a different filling system even though it is for a Vaillant boiler.
2. The valve you should be operating is the one with the black plastic handle as shown in your first photograph. It is attached to a chrome or nickel plated valve, not a brass coloured one.
3. Leave all brass coloured valves as they were before you started the exercise.
4. For the valve with the black plastic handle:
4a. Remove the handle by undoing the screw in the centre of the handle and pull the handle off.
4b. Underneath will be a round shaft with a flat on each side.
4c. Use a spanner, grips, or if you must, pliers to turn the spindle thorough 90 degrees.
4d. When the pressure is back to where you want it (around 1.2 bar is normal) turn the valve back through 90 degrees and refit the handle. The handle should be at right angles to the line of the pipe, as it is in your first photograph.
5. The fitting at the left hand end of the flexible pipe is a non-return valve. Should not need to be touched.