Interest-only mortgages are rare these days.
If you want to permanently switch to interest-only, you'll have to apply to your lender (that's if it even has the option). Where it does, you'll need to undergo an affordability check and – crucially –  prove you've got a credible repayment strategy in place to clear the balance when the mortgage expires.
There's no guarantee your lender will agree to a permanent switch to interest-only.