Assuming the sides of both pans are the same height (such as 2”), you divide the 9” circle area by the 8” circle area and that will give you factor to multiply all the ingredients by.
Another way to do it is to use pan volume: an 8” pan holds 7 cups and a 9” pan holds 8.67 cups, so 8.67 /7 = 1.24, so you multiply all the ingredients for an 8” pan by 1.24. Make sense?
I highly recommend the back section of the Cake Bible, where there is a system to formulate batters for any size pan. The system is more complicated than simple scaling, because for butter cakes leavened with baking powder/soda you also need to change the leavening when pan size changes to avoid doming or dipping.