You can only transfer funds between your Classic Account and ISA Account, for amounts that are in your Cash Balance. If you have funds which are invested/queued, then you will first need to sell/remove them.
To then transfer funds you need to navigate to your Accounts Overview page. You can do so by clicking on the Home icon in the top-left hand of the page.
You will then need to scroll down to reach the Account Transfer tool. All you then need to do is enter how much you wish to transfer, which account you want to transfer from/to and then click on Make Transfer.