How to update ipad that is only IOS 9.3.5?

Sorry no. If it says it's up to date, it means that that model (most likeky an iPad 2nd or 3rd generation or an iPad Mini 1st generation) cannot support iOS versions beyond the one that's currently running on it, because of hardware incompatibility.

Regards

Giulio

No.

Your iPad model is too old, now.

Sorry.

If you need/want an iPad with an up to date iPadOS with access to current, updated apps, then you really need to seriously consider a new iPad model OR a much “newer” , used iPad model capable of running the latest iPadOS versions. There are plenty of newer, older iPad models that can run the latest, current versions of iPadOS.

Sorry & Best of Luck to You!