There's no option to enter that information in step-by-step mode. To make that change to the tax return you will need to switch to forms mode. Please see the directions below.
Technically, if your father-in-law is incapable of preparing his own taxes, the return should be filed by his guardian or attorney in fact. If you hold one of these positions, you should also file Form 56 with the IRS. Please follow this link for more information.
https://www.irs.gov/pub/irs-pdf/f56.pdf
https://ttlc.intuit.com/replies/3339204