You can upgrade a KLM flight through the "My Trip" section of the KLM website or app, during online check-in, or at the airport via a self-service kiosk or KLM agent. KLM also offers last-minute upgrade offers, which are specific to a single flight and may appear during or after check-in. These upgrades can be purchased with cash, miles, or a combination of both, depending on the option. https://www.quickairfares.us/blog/klm-airlines-seat-upgrade/