Yes you can - providing that the vouchers have not yet been redeemed.
Here's how:
- Head over to your 'Voucher History' page (left hand tab).
- Find the relevant order.
- Scroll across to the right, click on the expiry date icon and select the new expiry date you wish to change to. Hit 'Apply'.
Please note: Recipients will not receive any notification that the expiry date has changed, but they will get a reminder to use their voucher - closer to the new expiry date.