Vouchers can't usually be extended, and the expiry date is the last date your voucher can be used.
If you're struggling to use a voucher before it expires, you're welcome to ask the venue nicely if they could give you a little more time. Please just keep in mind this may not be possible and they aren't under any obligation here.
You could also choose to gift your voucher to a friend or family member so they can enjoy it instead.