if exists (select * from ::fn_listextendedproperty ('MS_Description','SCHEMA','NCOS','TABLE','NCOS_Data','COLUMN','NC_Prospect_Name'))
begin
    exec sp_updateextendedproperty 
         @name='MS_Description'
        ,@value=N'Your description'
        ,@level0type='SCHEMA'
        ,@level0name='NCOS'
        ,@level1type='TABLE'
        ,@level1name='NCOS_Data'
        ,@level2type='COLUMN'
        ,@level2name='NC_Prospect_Name' 
end
else
begin
    exec sp_addextendedproperty 
         @name='MS_Description'
        ,@value=N'Your description'
        ,@level0type='SCHEMA'
        ,@level0name='NCOS'
        ,@level1type='TABLE'
        ,@level1name='NCOS_Data'
        ,@level2type='COLUMN'
        ,@level2name='NC_Prospect_Name' 
end;