To change a field's type in Marketo, please follow the steps below:
Log in to Marketo and press the Admin tab on the right upper corner:

Press on "Field Management" on the left bar:

Search for the field on the right bar and click on the field:

Press "Field Action":

Choose "Change Field Type."

Choose the required field Type

Press "CHANGE."

*Please note that a field with the type of "Score" can not be changed to any other type.