If you are entitled to a free upgrade due to your Avis Preferred tier, the app has been programmed to offer this automatically. If you are wanting to redeem a voucher for a free upgrade as a reward, then you will need to follow the usual voucher redemption process when making a booking by following the instructions listed on the voucher itself.
Don't forget to review the terms and conditions when you book, which include everything you need to know ahead of your journey.
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