What should be done to the papers uploaded on a preprint server, once they are published elsewhere. Should we remove them from the preprint server? Or what is the ethical thing to do
Here is a similar question with three answers which may help you: https://www.researchgate.net/post/If-a-paper-is-accepted-in-a-journal-is-it-mandatory-to-remove-it-from-arXiv . arXiv is a preprint server for physics, but the answers are valid also for other preprint servers.
I believe that the preprint is a confirmation of the date of your priority and in this context it should remain on the preprint servers after the official publication of an article or paper based on this preprint.