Two ways
1) If someone has SSC as a key feature on their set (usually the console) they treat it as a personal spead dial to make changes - press key, dial speed number, dial number to be added or changed.
You can list anyone with SSC in LD 81
Req: LST
Feat: SSC
You can print current entry in LD 20
REQ: Prt
Type: scl
LSNO: <-- enter list number for you system speed call
2) You can enter the changes at the system terminal in LD 18
Req: chg
Type: ssc
LSNO: <-- enter system speed call number
store: xxx yy..y <-- enter the speed call entry number and the number to be dialed by this speed call entry (021 910265551212)