I do not recommend to merge the preprint and the final versions. You can link the two pages (preprint and publication), see "How do I link my preprint to the publication page for the final version?" in https://explore.researchgate.net/display/support/Preprints. See, e.g., https://www.researchgate.net/publication/331546436 how this looks like.
If you would like to merge them anyway, see the section "Why are there duplicate research items on my profile?" in the help page https://explore.researchgate.net/display/support/Reviewing%2C+featuring%2C+and+editing+your+research to get instructions how to merge two entries. But I am not sure whether merging is possible in this case. I assume the two items should be of the same type, and it may not be possible to change from preprint to article. But you may try.
PS: I checked that it is not possible to convert a preprint item to an article item.
See also this paragraph in https://explore.researchgate.net/display/support/Reviewing%2C+featuring%2C+and+editing+your+research: "Please also note that we don’t merge articles with their linked data or preprints. We encourage our members to upload their work during all stages of the publication process, so we consider these to be separate research items."