My iphone 6 doesn't show software update under General. How should i update the software?

If an app or feature on your device is not working, it may have been disabled using Screen Time, or by an IT administrator if you are using a device owned by an employer or educational institution. See how to 'Use Screen Time on an iPhone, iPad, or iPod touch' by clicking here --> https://support.apple.com/HT208982

You can also see if you can effect an update by connecting to a computer:

"Update your iPhone, iPad, or iPod touch" click here --> https://support.apple.com/HT204204

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY

Finally realize that unless you are running an older version of iOS 12 as indicated in your device profile you may not be able to update at all. An iPhone 6 can only run as high as iOS 12.5.4

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting.      *

Restore - If you see an error when you update or restore your iPhone, iPad, or iPod.   *

Backup - Restore your device from an iCloud or iTunes backup.     *

In addition to trying the above, an iPhone 6 can't be updated past IOS 12.

See the article(s) below. 

iOS 14 and iPadOS 14 Compatible Devices.  *

iOS 13 and iPadOS are compatible with these devices - Apple Support.  *