The Questions tab allows you to change the question(s) type. Go to Questions tab:

Make sure the column Type becomes visible by adding them in the Column panel:

Above the Question grid, in the top right corner, you can open the Column panel used to select which columns to be displayed in the grid. The order of the columns can also be rearranged by drag and drop in the column panel.

When the column is visible, simply click on the type of the question you want to change and you will see the other option(s) appearing. When you’re done changing the types for your various questions make sure to click save at the top left of your screen. 

You can turn single, no scale questions into scale questions, so they can be used to calculate averages. 

You will not be able to change the type of an imported question in case they have been imported as an open variable; then they will stay open, and if they are imported as a categorical variable, they will stay categorical. To change this type of questions you need to import the data set again, and change the code and name of these, so you actually get two new variables in your data set.