how can i update my i pad MD518LL/A

10.3.4 is end of the line for that model https://en.wikipedia.org/wiki/IPad_(4th_generation)

if it does not run that then Update your iPhone, iPad, or iPod touch - Apple Support

Your iPad4 can be updated to iOS 10.3.3/4, this being the final supported version of iOS. It cannot be updated to later major versions of iOS/iPadOS as iPad4 does not meet the minimum hardware requirements.

Formal update support for iPad4 ended in September 2017.