This functionality is only available as part of the MobilityPlus eXperience with an EVdX subscription. (read here how to find your subscription type in the app)
Tap your charging station tile on the app start screen
In the screen that opens, go to your charging station settings via the cogwheel at the top right
In the screen that opens, tap your current energy contract
and tap Add new energy contract
Go through the questionnaire again to minimise the choice list of suppliers and possible contracts. Click here for more info
From the start date of the new contract, the new tariffs will be applied to the next reimbursement. It is not possible to make retroactive corrections.