In the Windows command prompt, type:

osql -E

Above
command should get you into MSSQL as administrator if you are logged
into the server as administrator(-E means trusted connection, i.e
Windows authentication). Then type:

sp_password NULL,new_sa_password,sa
go

where new_sa_password is your new sa password. The above SP, when executed, will change the SA password for you.

Then type

exit

to exit osql.

Now verify the password has changed by trying to login as SA with the new SA password.

osql -Usa