Just keep it in D(D-5) and it will shift automatically when it is supposed to.
You use D1-D5 when you are bored. Switching gears in an automatic car is more fun, but it is not required. You can just keep it in D-5.
You are supposed to shift everytime you reach around 3000rpm.
But shifting with an automatic transmission will hurt the transmission. You should just keep it on D-5.