I already filed my taxes, I need the plus feature but I accidentally clicked deluxe, how do I remove the deluxe so that I can pay for the plus version?

Unfortunately, since you have already filed the 2018 return, you can no longer clear it in order to downgrade to Free Edition with PLUS.    If you try to pay now, it will charge you for Deluxe Federal at 59.99 and possibly a state return at 44.99.   But here's what you can do:

You'll need to phone TurboTax Support, tell them you upgraded too far accidentally, and ask them to unlock the prior returns for you.    They may just unlock it for you without charging for it as a courtesy. 

Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Monday-Friday

FAQ: What is the TurboTax phone number?

https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632

I'd try that first, since they may not charge you.   There's also a free option if a return transcript will suit your purpose.   You may be able to immediately download one at the IRS website if you qualify to open an IRS account there.  Otherwise, to order a transcript to come by postal mail takes 5-10 calendar days.
http://www.irs.gov/Individuals/Get-Transcript