"I need to do to fix it" If your handy sure. If not Check YouTube . They have tutorials on everything. Audio/Visual. Do a search on the Marlin 60 loose bolt problem.
Or Marlin 60 problems. That type of thing.
How loose is it? It could be pretty lose and not be a mechanical problem ,however,too loose for me woul irritate me,is it loose enough to hear clatter when shooting? That would be really loose,and I have never seen one too loose to bother me,but I have seen some bad fitting stuff on Remins,wood and steel .you could order a new handle.