How do I change from turbo tax deluxe to turbo tax online for 2021

You can transfer from the prior year on your computer into the Online version. You will need to have the .tax2020 file on your computer. If you used the Desktop program last year here’s How to transfer into the Online version, expand the blue link if you used the CD/download last year........
https://ttlc.intuit.com/community/tax-topics/help/how-do-i-switch-from-turbotax-cd-download-to-turbo... 

Just start one of the 2021 online editions when available using your TurboTax account ID.

You can transfer the 2020 tax data file to the 2021 online tax return using the procedure shown in this TurboTax support FAQ - https://ttlc.intuit.com/community/entering-importing/help/how-do-i-transfer-last-year-s-return-into-turbotax-online/00/25953

And ps.....If you used the Desktop program before you may notice several differences and not like the online version. It has less features and no Forms Mode. You can only do 1 return. Your return is only saved online so be sure to save both the .tax2021 data file and the pdf file to your computer when you are done. In case you need to amend in the future or if you need a copy to get a loan or something. You might not be able to access your online return when you need a copy.

Did you mean change from the Desktop Deluxe program to the Online web version?  Online also has the same versions.  But in the Online versions you may need to upgrade to a higher version to enter certain items.  Like you need Premier to enter investments, sales and rentals.