If you or your spouse is 65 or older. The 1040SR is exactly the same as 1040. Just bigger type. You can only change the form using the Desktop program.
If you want to use the regular 1040 then print out your return and copy the amounts onto the 1040 from the 1040SR.
1040SR
https://www.irs.gov/pub/irs-pdf/f1040s.pdf
1040
https://www.irs.gov/pub/irs-pdf/f1040.pdf
