Hello,
You cannot change it directly but a new purchase and refund is possible. I suggest you buy your Swiss Travel Pass for 4 days and then ask for a refund for the previous pass here. If you have any doubt, you can call us at 0848 44 66 88 (CHF 0.08/Min) or +41 51 225 78 56 (from abroad) or contact us via our live chat. Note that there is generally a CHF 10.00 deductible for the refund.
I wish you an amazing stay in Switzerland,
Chiara_S