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

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.

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.