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You need to delete the rendered files and then you can get rid of the trial watermark.
However, if the video has already been exported that cannot be removed.
The reply that GautamBahl is the classical answer to how to remove the Adobe watermark from the trial version project file after you open the trial version project file in the purchased program.
However, you may not have to do that. Many of us have found that just opening the trial version project file (with Adobe watermark) in the purchased program automatically removes the Adobe watermark. If that does not work for you, then the classical way using deletion of the rendered files (Expert workspace/Timeline Menu/Delete Rendered Files).
Thanks a lot. it was very helpfull.