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.