I want to update my iPhone 6 to iOS 14

KashifAhanger wrote: "But it's too much lagging what should i do"

If the battery of your iPhone 6 is the original one, perhaps it needs replacing. "Lagging" may be a "performance management" effect by iOS to prevent unexpected shutdowns due to battery age:

• Longer app launch times
• Lower frame rates while scrolling
• Backlight dimming (which can be overridden in Control Center)
• Lower speaker volume by up to -3dB
• Gradual frame rate reductions in some apps
• During the most extreme cases, the camera flash will be disabled as visible in the camera UI
• Apps refreshing in background may require reloading upon launch

[Source: iPhone Battery and Performance]

iPhone Battery & Power Repair

You can find Apple Support Articles such as the above by searching Apple's Knowledge Base.

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1. A Reset may cure some bugs. Hold down the Home and sleep/power buttons until the apple logo shows up.
   1. If still not resolved:
2. Reset all Settings. Go to Settings->General->Reset->Reset All Settings
   1. If its still not resolved:
3. Restore the device as new.
   1. http://support.apple.com/kb/ht1414

It can't run iOS newer than 12.5.

That phone cannot update past iOS 12.

Sorry, you cannot have it.

The iPhone 6 cannot run iOS 13 or 14. It simply does not have the hardware resources to run them.