My phone doesn’t show an available update but its still ios 12

if you are trying to update to iOS 14 and your phone is showing that it's up to date.. and you're running iOS 12 (probably 12.4.9), then your iPhone is a 6/6 Plus or older and cannot be updated any farther than it already is

If you have an iPhone 6 or 6s that’s all the higher it will go.

What model is your phone?