How do I replace an iPhone battery that won't hold charge?

Make an appointment at the Apple Store / Genius Bar or take the phone to an authorized Apple service shop to have the battery replaced.

You can find a list of authorized Apple service shops using this link: Find Locations

Give the shop a $100 bill and the change might buy you a cup of coffee.

If you are the DIY type you can order parts and rent tools from Apple to replace your battery yourself.

Self Service Repair - Apple Support

Your profile indicates you have an iPhone 14 Pro. If that is true then your battery should have lasted longer than two years. Go to Settings->Battery->Battery Health and report back what you see there.