How I re-install ios 11.2.6

To install a fresh copy of current iOS version on your iphone, make a full backup of your iPX:

How to back up your iPhone, iPad, and iPod touch - Apple Support

Then follow the steps provided in this support guide, and, when finished, set it up as new:

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

When done, restore the iPhone X from your previously made backup:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Regards

Giulio

Connect your phone to a computer with iTunes........open iTunes and click your phone logo located upper left......click restore.