iPhone 6s out of storage to update.m to 15.7.6

Here are some helpful articles that can guide you on how to free up storage space on your phone.

You can also update using a computer, it requires less space on your phone this way.

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

 How to check the storage on your iPhone and iPad - Apple Support

Manage your photo and video storage - Apple Support

 Manage your photo and video storage - Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

 Clear Other storage on your iPhone or iPad - Apple Support

Update using your computer -

Click on this link for instructions on how to update your phone using a computer

Update iOS on iPhone - Apple Support

You're welcome, good luck.