You can move money between a Cash ISA and Stocks & Shares ISA in Settings > Move money between accounts.

Moves between ISAs do not count towards your annual £20,000 allowance.

If moving funds from a Cash ISA to a Stocks & Shares ISA, please note that this will use one of the three Cash ISA withdrawals you can make in a 12 month period before impacting your interest rate, and if your Cash ISA balance drops below £500 you’ll earn a lower rate of 0.75% AER (variable).

If moving from a Stocks & Shares ISA to a Cash ISA, you can only move funds held in your Available Cash balance. If you don’t have any Available Cash, you can add cash by selling down your investments in a specific fund under Accounts > S&S ISA > Tap your individual fund(s) > Sell.