It is fairly simple.
Remove the engine shroud. The dipstick tube should come out when you lift the shroud. If not then you should be able to just pull it out. (Once in a while there might be a screw that holds the tube in place).
Then remove the starter mounting screws and replace the starter.
Sorry, many of these engines have different ways of removing the tube. Just a little different modle calls for a different procedure. Many times you have to remove the shroud just to get to the bolts that hold on the tube. Also sometimes it is much easier to get to the starter bolts if the shroud is out of the way.
At least you found out that you had to remove the tube. And Hopefully you were able to remove the starter. If you need any more information, please let me know. You can still contact me here at this question.