Login as administrator
Go to su01
Enter the user to whom you want to change the password
There will be a icon called change password. Click it.
Enter a new password twice.
If you have Central User Administration perform the SU01 password change from there.
Once you have clicked the change password icon you will see all target systems are selected. Un-select any target systems you do not need to change the password for.
Continue with entering the new password and click on the green check box at the bottom.