Couwhiemboo57 Hello
Do you mean a Swiss Half Fare Card valid one month? If the card is already valid, it is not possible to change the dates.
If the card is not valid now (start date in the future), you could obtain a refund in the app and buy a new one with the correct dates.
Or call the Contact Center of the SBB:
0848 44 66 88 (24/7, waiting times may occur)
From a foreign country the number is: +41 51 225 78 56