How am going to update my iOS version

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. 

An iPhone 6 isn't capable of 13 or above. To update it to 12.5.5, use a computer or the Software Update function in Settings.

(221868)