Heroes past level 5 by creeping
Normally heroes stop gaining experience when they have reached level 5, but it's actually very easy to change those limits.
Go to "Advanced" -> "Gameplay Constants"
Tick "Use Custom Gameplay Constants"
Find the field called "Hero XP Gained - Creep Reduction Table"
This is the table where you can see how much experience you get on each level. As default it's set to "80,70,60,50,0" which we can put in a table like this:
1 - 80%
2 - 70%
3 - 60%
4 - 50%
5 - 0%
So by changing the 5th limit to 40% instead, you will be enabled to reach level 6. Then at level 6,7,8,9,10 you will still receive 40% experience so you might want to add more values to it.
Heroes past level 10
In many WarCraft scenarios you can now raise a hero's level to above level 10. This is done by changing one single value.
Go to "Advanced" -> "Gameplay Constants"
Tick "Use Custom Gameplay Constants"
Find the field called "Hero Maximum Level"
Set this to whatever you want the maximum level to be.
NOTE: Maximum is 10,000.