1040SR

If you prepare a tax return you are preparing a Form 1040.    If you are 65 or older, when you print the return later it will print out on 1040SR.   Form 1040 and Form 1040SR are identical.   The only difference is that 1040SR has bigger print.  All of the lines are the same and all of the calculations are the same.  Your tax due or refund are not affected in any way whatsoever by 1040 vs 1040SR.

As long as you meet the qualifications for 1040-SR it is automatically applied to your tax return. Form 1040-SR (U.S. Tax Return for Seniors) is simply a larger-print version of Form 1040 for taxpayers age 65 or older. The same lines, schedules, and calculations are used.