You will need to merge the two gedcom files in one of the many software tree programs that is available, then upload that merged gedcom file to TribalPages to replace the one you have already uploaded for your tree.
If the first file you uploaded has been on TribalPages for a while, and you've added new information to that tree, that new information will be lost with the new merge upload. You will need to download that existing tree on TribalPages to a new gedcom file and then merge with that second gedcom file as mention above.