How do i file my extended 2018 taxes

How do I finish a prior-year return that I started in TurboTax Online?
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-finish-a-prior-year-return-that-i-started-in-turbotax-online/00/26322

You can only buy the download for prior years. You have to buy a separate program for each year. They only sell the last 3 years. You need a full Windows or Mac computer to install it on. You can buy any version. All the Desktop programs have the same forms. You just get more help and guidance in the higher versions. Buy the download here
https://turbotax.intuit.com/personal-taxes/past-years-products

If you want to file for free you can fill out the forms by hand.

Here is the IRS 2018 Form 1040
https://www.irs.gov/pub/irs-prior/f1040--2018.pdf

And 2018 Instructions https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf
2018 Tax and EIC Tables https://www.irs.gov/pub/irs-prior/i1040tt--2018.pdf

Sch 1 : https://www.irs.gov/pub/irs-prior/f1040s1--2018.pdf
Sch 2 : https://www.irs.gov/pub/irs-prior/f1040s2--2018.pdf
Sch 3 : https://www.irs.gov/pub/irs-prior/f1040s3--2018.pdf
Sch 4 : https://www.irs.gov/pub/irs-prior/f1040s4--2018.pdf
Sch 5 : https://www.irs.gov/pub/irs-prior/f1040s5--2018.pdf
Sch 6 : https://www.irs.gov/pub/irs-prior/f1040s6--2018.pdf