Yes, the interest rate on your Save account can change.  You can find the out what the current interest rate is by opening the Stream app and going to Hub -> Save.  The interest rate will show under the balance amount at the top of the screen.

The money in your Save account is held by Investec Bank Plc.

If Investec increase the interest rate on your account, the change will take effect on the first day of the next calendar month (or in some cases, immediately).

If Investec decrease the interest rate on your account, Stream will notify you by email at least 14 calendar days before the decrease happens.