Sorry you can't apply the Desktop purchase to Online.
Why do you want to switch to the online version? Online is different than the Desktop program and has less features and no Forms Mode. And you might have to upgrade to a higher version to enter something. In the Online version you can only do 1 return. What problem are you having in the Desktop program?
To use the Online version you would have to start over in a new online return and pay for Online separate from the Desktop program. Then try to get a refund for the Desktop program.
Start an Online return here,
https://turbotax.intuit.com/personal-taxes/online/