Yes you can - providing that the vouchers have not yet been redeemed.
- Head over to your 'Orders' page.
- Click into the relevant batch of vouchers.
- Click on the 'Order detail's dropdown.
- Next to the expiry date, click on the pencil icon.
- Select the new expiry date and hit 'Change'.
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.