How do I start a second return using the Download version of the software?

Just go up to File - New Tax Return.  It will ask to save your return.  Also be careful if your name is the same as another return. Turbo Tax saves the tax returns as 2024 Smith J Form 1040 Individual Tax Return.tax2024. So if you have two or more returns for people that have the same last name and first initial the second return will overwrite the first one. So you have to be sure to first save the returns with names completely different to keep them apart. Don't use the default name given by the program. You need to go to FILE - SAVE AS and give the return a unique name you can tell apart. At least add the full first name. You don't have to use or accept the name Turbo Tax gives it. I save all my returns with different names.

Are you on Windows or Mac?  Do you need a screenshot?  Here's my Windows