groupmod – Modify a group 

groupmod [-g gid [-o]] [-n group_name group 

The groupmod command modifies the system account files to reflect the changes that are specified on the command line. The options which apply to the groupmod command are

-g gid
The numerical value of the group’s ID. This value must be unique, unless the -o option is used. The value must be non-negative. Values between 0 and 99 are typically reserved for system groups. Any files which the old group ID is the file group ID must have the file group ID changed manually.
-n group_name
The name of the group will be changed from group to group_name.