Update your iPhone, iPad, or iPod touch - https://support.apple.com/HT204204 - When you try to update you will only be given one update selection which will be the highest available iOS version supported by your device. If it says no update is available and you need a higher version you will have to buy newer equipment. Note, this method is how you yourself can find out what the current highest available version is for any device you own without us providing a list.
Since you say "9.3.6" which is a very recent release, that suggests to me this is a model which cannot be updated past the present version.