I am new and as a result I a field Multiple Choice instead of a single choice, How do I change the field type?

Under the Actions pulldown menu, select "Edit field". Drag new field to center column. Delete old field. The fields will appear on the form layout in the same order as they are listed in the center column. You can drag and drop the fields to change their order.

Yes, if you delete a field you will loose that data. To my knowledge there is no way to just change a field type.

You can create a new field. Use the original field to populate the new field with the data then delete it. Then if need be, rename it. If you had any code referencing the old field elsewhere it will of course fracture.

Maybe this can help:

https://ninox.com/en/forum/use-cases-5abd0b9c4da2d77b6ebfa395/how-to-convert-a-multiple-choice-field-to-a-choice-field.-5f36a96d539bd77f590ca699?post=5f36a96d539bd77f590ca69a&page=0

Steven