If you've rendered the timeline or output a movie from the trial version, it will have that watermark on it.
Once you've purchased and activated the software, you can remove the watermark by going to the directory folder on your computer where your project file(s) are being saved and deleting the Adobe Premiere Elements Preview Files folder.
Once you activate the product after providing the serial number (this should happen in the background after entering the serial number) and then you open the project file (.prel file), all the watermark shown anywhere inside PRE should go away.
Is that true for already rendered timelines, Tahseen? If preview files have already been created, they will still have the watermark on them until they are deleted or recreated, right?
That's right Steve, already rendered files will need to be deleted and created again as those are physical files created and watermark has been embedded in that.