When I get the software update it keeps saying software is up to date with ios 12.5.5 how can I get either 13 or 15

I can't afford to get another iPad or computer and when I try to use my iPad to order groceries at my local HEB it keeps telling me I need to update my data/ios so I am just screwed all the way around, being handicapped and only on a very limited social scurity income {thanks to the politica stats} Many blessings and thank you for the info.