How can I update iOS on my iPhone 13 Pro Max

Please 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. 

Follow the instructions:

Update iOS on iPhone - Apple Support

If you are looking for iOS 18, that will not be available for another 8-10 weeks.

What happens when you follow the advice posted above?

What is the existing version of iOS?

You're on 17 which is the latest version, 18 won't be out for a month or so.