Based on iPhone

Hello

Since it looks like your iPhone is compatible with the lates OS, and you’re still at 13.4, you should probably start updating your phone a lot to get to the latest OS, since the iPhone 6s is compatible with iOS 15. Just update your phone like you usually do for iOS 13.4 (Settings>General>Software Update).

Hope this helps.

If you have an iPhone 6s, you cannot update to iOS 13.4.....but....you can update to iOS 15.6

Update your iPhone, iPad, or iPod touch - Apple SupportOfficial Apple Support › en-us

Yes I have iPhone 6s

Please click on the link that I provide above (and below) and follow the instructions to update the software on your iPhone.

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