Edit cookbook or role on your chef repo. Don't forget update its version.
% knife cookbook upload [cookbook]
% knife role from file [file e.g. roles/ssh_known_hosts.rb]
Dry run at first:
% knife environment from file environments/prod.rb --dry-run
Then do it:
% knife environment from file environments/prod.rb
% knife node run_list add node_name "role[rolename]"