If you've not set up a mortgage: From Dashboard (aka 'Home') > Debt to add a mortgage plan and choose the details.
At 'Interest Type' click on 'Fixed Interest'. This enables you to enter numbers next to the Fixed Period Term.
Use the Remaining term for the variable portion of your loan and click Save plan.
If you've changed loans (eg was variable and now fixed): In your existing debt plan ie Home, scroll down the page to see a drop-down menu called 'Debt details'. Click on the words and the menu will open where you can change Interest types.
Got split loans with different lenders: Create 2 Debt plans per lender and choose the details.