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."). I recommend to keep the preprint and to add the final version as a new entry. 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).

See also these similar discussions:

https://www.researchgate.net/post/Can_I_delete_a_preprint_manuscript_before_publication

https://www.researchgate.net/post/How_to_Replace_a_publication-update_preprint