I think I'm correct in saying that with Garanti, you can close or withdraw from an exsisting time deposit account online, only if you opened it online. If you opened it in branch, you can only close or withdraw from it in branch.
You can then open a new time deposit account online for 29 days.
Remember, you must wait till your maturity date to change anything, otherwise you will loose your months interest.
I'm sure if you Email Garanti, they would be able to make the changes for you.
Also, these interest rates are for balances over 25,000ytl.
One other thing, although 29 days gives a good interest rate at the moment, that may not be the case next month..you need to review the rates every month on your maturity date and change the interval if necessary.
It realy does pay to keep an eye o these things.