Navigate to src/main/java and rename the package and the .java File to your liking.
Suggestion: "me.yourname.yourproject" (all lower case) and "ProjectName.java"
Example: "me.thebusybiscuit.cooladdon" and "CoolAddon.java"
Navigate to src/main/resources/plugin.yml and change the "author" and "main" attributes.
You may also want to change the description to something meaningful.
Navigate to pom.xml and change the group id to "me.%Your name%" and change the artifact id to the name of your Project.
After that you are good to go, you can now start developing your own Addon for Slimefun4.