Update the List item permission programmatically

Suppose if you have already bind a sharepoint group to a list item in a custom list/library for integrating permission then later on , if you want to modify the permission for that list item, below is the code snippet to achive it.

SPRoleAssignment objRoleAssignment = objListItem.RoleAssignments.GetAssignmentByPrincipal(objSPGroup);
objRoleAssignment.RoleDefinitionBindings.Remove(web.RoleDefinitions["Full Control"]);
objRoleAssignment.RoleDefinitionBindings.Add(web.RoleDefinitions["Contribute"]);
objRoleAssignment.Update();

Samarendra Swain