Extra storage

Is this about iPhone storage or iCloud storage?

If it is about iPhone storage, then sadly, no, you cannot upgrade your iPhone storage, but you can buy and external drive to store more of your files on, or you can use iCloud Drive. For instructions on that, look below.

If it is about iCloud storage, go to Settings > Your iCloud Profile > iCloud > Manage Storage > Change Storage Plan. Note that 2TB is the absolute maximum, you can't go higher than that. If you want to, then buy your own server for use at home. There are plenty out there for home use.

The same way you buy it. When buying extra storage you select a a storage size option. When you want to cancel it, just select the free 5GB option in the same place.

See here: Buy more iCloud storage - Apple Support

By changing it at the same place that you buy it.