Hi Rthomas,
Could you please share the errors in your flow, so that we could take a further investigation?
I would say I also stucked at the file path part. But got this one working.
See the details from my testing:

Based on what I have tested, when selecting the DropBox available function "When a file is modified", it will only accept a file path, so if we would just want to update one file, then please specify a special folder for the file in DropBox, after that, we could click on the folder icon, and navigate to the proper folder;
After that, click to add an action, choose SharePoint "Update File", enter the site URL in the "site URL" box, then under the File identifier, click on the folder icon and navigate to the File that you would like to update (Well, I have wrongly choose the File path from the auto-generated content in DropBox, which won't work),
After that, under the File content, we could choose the File Content, click save.

Please test this one and see if it would work for you. Or if you need any further assistance regarding file update, please feel free to post back.
By the way, there are templates available to Copy the file between DropBox and SharePoint,vice versa, see:
Copy file between Dropbox and SharePoint
Regards