Why am I being taxed on interest I haven't earned yet

I received a change of tax code letter in January saying that it had been estimated that I will owe tax on savings interest for the current tax year so my tax code was changing. This may be possible but until I receive my end of year statements I won't know for sure but I shouldn't because most of my savings are in a cash ISA. I have now received another tax code because it is estimated that I will owe tax on interest for next year. I haven't earned this interest yet but tax will be taken, plus, I will definitely not earn enough interest to put me over the £1000 allowance as I carefully manage my accounts. How can I get this changed as I should not be paying tax on money I am not earning.

Hi,
This forum is for general queries only and is intended to help you self-serve.
We are unable to provide specific advice tailored to individual circumstances. 
It is not possible to check individual tax records through this forum, if you have a question which relates to your personal tax situation or tax code please contact our helpline.
Thank you.

I would like to self serve. Also it states in the tax code letter that if the tax code is wrong to let HMRC know of any changes on line via your personal tax account, however, there appears no way to actualy do this unless I am missing something. I don't want to spend an hour on hold to get this changed.

Hi,
Please see the following link on how to update to your taxcode.
Tax codes: How to update your tax code
Thank you.