Upgrading to Premium Economy, Business or First Class depends on seat availability and the type of fare that you currently have for your booking.
The airline often charges additional fees, which may include a change penalty and any difference in fare from the time you booked to today's price, if upgrades are available. Depending on the airline, Hopper may be able to provide you with a price quote. If not, you'll need to reach out to the airline directly for further assistance.
Using Frequent Flyer Miles to Upgrade
Unfortunately, as an online travel agency, we cannot accept frequent flyer miles to process an upgrade on our end. These must be exchanged directly with the airline you have an account with or with your credit card company.