Hi Andrej,
I understand that you would like to reverse the direction of the DC motor in the MathWorks "DC Motor Model" example.
To reverse the direction of the motor, you simply need to reverse the output polarity of the H-Bridge motor driver. You can do this by changing the voltage at the REV port of the H-Bridge. see the H-Bridge documentation for more information: http://www.mathworks.com/help/physmod/elec/ref/hbridge.html?searchHighlight=rev
For controlling the voltage at the REV port, you can use a Controlled Voltage Source block. See the block's documentation for more information: http://www.mathworks.com/help/physmod/simscape/ref/controlledvoltagesource.html
