Hi..
First, You should remove cost center group from used
lists(ex:allocation cycle, allocation structure, transfer structure, splitting structure, report layout ,etc)
of cost center group.
If cost center group is group of standard hierarchy , you delete or
change cost center group using t-code OKKEN.
If cost center group is group of non-standard hierarchy , you delete or
change cost center group using t-code KSH2.
If cost center group of non-standard hierarchy have
group of standard hierarchy as lower group, you remove standard
hierarchy group from non-standard hierarchy group and delete
non-standard hierarchy group and delete Standard hierarchy group.