How can I update my iPhone 6 to a better iOS version?

To update to the most recent, compatible version of iOS 12.5.7, 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. 

Your iPhone is too old and would not be compatible with the newer software updates.

Click ▶︎ iPhone compatibility with iOS 13 for the complete list.

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Sudrex50 wrote:

how will I update my iOS to a better version?

By getting a newer phone if you want the newest ios, your current phone is outdated and can't get the latest ios.