In our Dynamics CRM online instance, we've got the Notes
entity:
which allows file uploads:
We're able to change the label using Form Editor
We just need to update the label NOTES above the text box and No Notes found. below the text box.
Instead it should be DOCUMENTS and No Documents found.
Create a solution and add the Notes entity to it.
Then, Click on the Export Translations button in the solution.
Open the exported file and Update the translation text of Note and Notes.
Save the file and re import the translations file.
This will change the text in all the forms of your organization, not only the form you are customizing.
If you want to change the text in only one form, the only way I see is use an unsupported Javascript to find the html element containing the text and change its values.