How do I clear/buy storage to update macOS on MacBook Air?

Which MacBook Air do you have – i.e., what does it say about the hardware model year in  > About This Mac ?

That will determine

• How far you can update macOS
• Whether it is possible to purchase aftermarket internal SSD upgrades for that machine

Your tag line says that you are running Mojave. There are no MacBook Airs that are compatible both with macOS 10.14 (Mojave), and with macOS 15 (Sequoia). Best case is that you have a 2018 or 2019 model – which you'd be able to update all the way to Sonoma. Sonoma is one of the "most recent three" and will be even after macOS 26 Tahoe comes out this fall.

Worst case would be a 2012 – 2017 model that cannot update past Catalina, Big Sur, or Monterey (depending on model year). All break 32-bit applications. None are still among the "most recent three" supported by developers like Adobe and Microsoft. They are recent enough to run current versions of Firefox, LibreOffice, the GNU Image Manipulation Program (GIMP), and the commercial Affinity V2 applications (Photo, Designer, Publisher).

Update macOS on Mac - Apple Support

As for aftermarket SSD upgrades, OWC has them for many MacBook Airs – but not for ones with hardware model years of 2018 and later. Apple started soldering in SSDs in 2018, making internal upgrades impossible.

Other World Computing – MacBook Air – Solid State Drives

So there's no possibility to upgrade internal storage and update to one of the "most recent three". At best, you'll have the option to do one or the other.

You will then need to delete items from your Mac or offload files to an external drive to free space.