Greetings Cadet:
Technically speaking, you don't become a commissioned officer (O-grade) until after graduation from the Academy, so your E-5 designation would be more correct. That said, it really will make no difference which military rank you choose (or input) into the TurboTax program with respect to the actual tax return that will be filed with the IRS (and your state department of revenue, if applicable). In other words, officers are taxed on their military pay, as are cadets on the taxable portion of their compensation, just as are enlisted military personnel. All are typically equal, generally speaking, under the eyes of the IRS.
That said, there are a few exceptions to this policy, such as how combat pay is treated (i.e., tax free), how the states treat soldiers and their spouses stationed outside of their home state, and a few other tax "oddities." But in your case, there should be no difference at all between an O-1 and an E-5 input in the personal information section of TurboTax, with respect to your actual tax return.
There can be a difference in the price charged by the TurboTax program (if any) for creating your tax return. But otherwise identical O-1 and E-5 tax returns will come out to the very same tax results in the end . . . in terms of a refund or amount owed (liability).
If this becomes a problem, you always have the option to open a new online tax account and start over, or to "clear" your account and start fresh. Here is a TurboTax FAQ page on that:
https://ttlc.intuit.com/questions/2586254-how-do-i-clear-and-start-over
Thus, please don't feel that you've necessarily "messed up" your tax return in any substantive way.
Additionally, you may or may not be aware of the free tax preparation services otherwise available to you at the Academy itself. You may wish to avail yourself of this. Here are some links to more information about that option (presumably you know the administrative buildings and the layout on campus to find them):
http://www.usma.edu/sja/SitePages/Tax%20Center.aspx
http://www.usma.edu/parents/siteassets/tax%20letter%20for%20cdt%20%20parents.pdf
I hope that you find this information helpful; and thank you for asking about this. Finally, on behalf of the many veterans and non-veterans here at TurboTax, we thank you for your military service.
