I want to update my iPhones to ios15.5

Hello ~ Not sure if you still need help as you marked you post solved but just in case…

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

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

~Katana-San~

iOS 15.5 for iPhone 6splus

Settings -> General -> Software Update

It is this easy, just be sure to have enough free space to install the update