You cannot "upgrade" a 10kg bag to a 20kg bag by simply paying a small difference; instead, Ryanair's 2026 policy requires you to purchase a new 20kg checked bag allowance. If you have already purchased a 10kg checked bag and realize it is too small, you can add a separate 20kg checked bag to your booking through the "Manage Trip" section of the Ryanair app or website. Note that the 10kg bag you already paid for is non-refundable. Alternatively, if you arrive at the airport and your 10kg bag weighs more than allowed, you will be charged an excess baggage fee per kilogram (usually around €9–€11 per kg), which is significantly more expensive than pre-booking a 20kg bag online. If your 10kg bag is meant for the cabin (Priority & 2 Cabin Bags), you cannot increase its weight limit to 20kg at all, as the overhead locker weight limit is strictly enforced for safety. Your best option is always to weigh your bag at home and add the 20kg allowance online at least two hours before your flight to avoid steep airport fees.