How to update if my ipad is 10.3.1

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Update your iPhone, iPad, or iPod touch - Apple Support

if it say it's up to date then it's because the ipads hardware do not support a newer version of ios.

To update your iPad, go to settings>General> and software update. If you are using an iPad 4, iOS 10.3.3/10.3.4 is the latest version that it supports.