Simple steps:
1. go to admin mode
2. check the running-config for username section
3. config and edit the username &/ password as you want
4. verify/check again the running-config for username section
5. exit from admin mode
Sample:
>>> 1. Go to admin mode
RP/0/RSP0/CPU0:ROUTER#admin
RP/0/RSP0/CPU0:ROUTER(admin)#
>>> 2. check the running-config for username section
RP/0/RSP0/CPU0:ROUTER(admin)#sho run username
username root
group root-system
secret 5 $1$shwg$h/PpC.
!
username cisco
group root-system
secret 5 $1$9P5U$Kz0E41
!
>>> 3. config and edit the username &/ password as you want
RP/0/RSP0/CPU0:ROUTER(admin)#config t
RP/0/RSP0/CPU0:ROUTER(admin-config)#username new-root-admin
RP/0/RSP0/CPU0:ROUTER(admin-config-un)#group root-system
RP/0/RSP0/CPU0:ROUTER(admin-config-un)# secret passroot
RP/0/RSP0/CPU0:ROUTER(admin-config-un)#exit
RP/0/RSP0/CPU0:ROUTER(admin-config)#no username cisco
RP/0/RSP0/CPU0:ROUTER(admin-config)#commit
RP/0/RSP0/CPU0:ROUTER(admin-config)#end
RP/0/RSP0/CPU0:ROUTER(admin)#
>>> 4. verify/check again the running-config for username section
RP/0/RSP0/CPU0:ROUTER(admin)#sho run username
username root
group root-system
secret 5 $1$shwg$h/PpC.
!
username new-root-admin
group root-system
secret 5 $1$w8YNsFS.pyQ.
!
RP/0/RSP0/CPU0:ROUTER(admin)#
>>> 5. exit from admin mode
RP/0/RSP0/CPU0:ROUTER(admin)#exit
RP/0/RSP0/CPU0:ROUTER#
RP/0/RSP0/CPU0:ROUTER#
