First, back up before renaming in case something goes wrong.
Usually when someone wants to rename the project, what they really want to do is rename the app. You can do that by renaming the target. Renaming the target is safer than renaming the project.
Select the project name from the project navigator to open the project editor.
Select the target from the target list on the left side of the project.
Press the Return key.
Enter the new target name.
If you really need to rename the project, select the project name from the project navigator and press the Return key. The following article has more detailed instructions: