How can I fix my iPhone 15 Pro overheating issue?

To add to what lobsterghost1 said, there are a number of different reasons for the iPhone to get warm, charging, weak cellular signal, playing games, watching video, anything that requires a lot of processor power and/or graphics power. For a day or so after updating iOS you will also see the device doing a number of housekeeping tasks to re-index files, etc. All of those will lead to the phone getting warm. Without a fan on the device, it dissipates heat through the case.

This is not Apple Support. This is a user to user only forum. We can't fix anything on your iPhone.

When you say overheat, are you actually seeing a thermometer on your iPhone screen telling you the phone can't be used until it cools down? If no, your iPhone is not overheating. iPhone has thermal protection built into it to protect from overheating.