Changing the SQL Server Password

All of these failed except the last one, I was logged in as RMUser:

exec sp_password 'DC9E3622', 'A55BE7!#4', 'RMUser'

Msg 15210, Level 16, State 1, Procedure sp_password, Line 20

Only members of the sysadmin role can use the loginame option. The password was not changed.

ALTER LOGIN RMUser WITH PASSWORD ='A55BE7!#4'

Incorrect syntax near 'LOGIN'.

exec sp_password @old = 'DC9E3622', @new = 'A55BE7!#4'

Command(s) completed successfully.