I want to install iOS 15

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

sarab ~ Does your iPhone 6s still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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