You will have to purchase the CD/Desktop version of Turbotax 2018 and install it on your computer. You can get it at https://turbotax.intuit.com/personal-taxes/past-years-products/
After you pay for it, you can download the CD installation file and install it immediately so you don't have to wait for the CD to arrive in the mail.
Take note that a prior year tax return can not be e-filed. The IRS says so. You will have to print, sign and mail it to the IRS. Mailing instructions will be included in the printout. If you are required to file a state return, then seperate maililng instructions will be printed for the state return.
