How can I upgrade battery capacity

You can't, batteries are consumables the chemicals in them degrade over time reducing the battery health.

ozioma272 Said:

"How can I upgrade battery capacity"

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Update for a Recalibration Fix:

Perhaps the calibration is wrong, and and you need to install an update to your iPhone that fixes this? So, update it. For more, see the link provided atop this post: About recalibration of battery health reporting in iOS 14.5. 14.6 is the latest update; both would be installed. Go here to update it: Update your iPhone Using a Computer - User Tip