It is not possible to merge these, because it is not possible to change the type from "Preprint" to "Article", see https://help.researchgate.net/hc/en-us/articles/14293081125777-Reviewing-editing-and-featuring-your-research ("Certain types of research cannot be changed to another type, including data and preprints. To change the type, you will need to remove the research item and reupload it as the correct type."). ResearchGate recommends to keep the preprint page and to link it to the page with the final (published) version. You may connect both items by a link as described in the last paragraph of the help page on preprints (https://help.researchgate.net/hc/en-us/articles/14293104044177). This link will look like in my preprint page https://www.researchgate.net/publication/331546436 (the link is between title and authors). In addition, you may also use the "Comments" fields to connect both pages, see my example.