Complete level 6 sfeb5h6bgtegbu
it is not stars it is a world
in world 1 at the tower there is a red flag that you need to get to unlock the cannon.
I think you mean how to unlock it. -Either complete World 5 by defeating the boss with MINI MARIO. -Or unlocking the cannon on World 4.
you can unlock a level with a pipe and cannon around it (i got it on a red flag on world 2 number 4 ) and finish that on a cheat flag( the yellow pipe above the other 1). this will unlock the cannon.
There is no cannon on world 1
look up super Mario bros Cannon world 1 and level 1
kill mario
The cannon in new super Mario bros takes you from one world to another without having to complete the world (ie it shoots you into another world)
By going into a cannon
get to the cannon lvl
Go into the first castle and when you get to the partwith the 4 rows of falling blocks run and jump at the black part of the left wall.
World 7