chown USERNAME FILENAMEand press the ENTER key. To change the group type:
chgrp GROUPNAME FILENAMEand press the ENTER key. If you want to change directory owner / group (recursively) type
chown -R USERNAME ./FOLDERNAME
chgrp -R USERNAME ./FOLDERNAME
Note that, since the Unix-like systems are case-sensitive, the -R flag must be entered in uppercase.