My phone says its up to date when its only on ios 12.4 how do i update it

This is very strange, as iOS 12.4.1 was released before iOS 13, so iOS 12.4 is not current for any device.

Might your iPhone be restricted? Check Settings > General > Screen Time

Might something else be going on you’re not telling us? Perhaps your storage is full, or you’ve changed country...

Check if you have a beta profile for ios 12 installed under Settings>General>Profiles. If so, delete that profile. My girlfriend had this issue and it resolved the problem.