At present, you cannot change the type of your existing portfolio in the app or on the website.
If you wish to change between DIY and Managed, or ISA and GIA you will need to create a new portfolio and initiate a transfer of funds from your old portfolio.
Please make sure to transfer your funds internally as described here. Withdrawing funds from one portfolio and depositing them into another via your bank account may result in losing part or all of your ISA annual allowance.