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Unfortunately, you cannot change the type of the field in your form.
In this case you'll have to add the checkbox to your form.
Please let me know how to do that without losing all of my data.
I suggest that you hide the radio button. This will allow you to keep the captured data from this field and it will not be shown in your form once is hidden.
Right-click it and click on Hide Field.
Please let us know if you have further questions.