In World Editor, click on "Advanced", click on "Gameplay Constants", lastly click on checkbox "Use Custom Gameplay Constants".. with that you can edit hero level and experience.
Look for "Hero Maximum Level", change it to 80.
Look for "Hero XP Gained - Creep Reduction Table" those makes you stop at level 5 from creeps (with the last number 0) You can change delete, add, change numbers, and last number (higher than 0) will keep you experiencing until 80.