Once the data type is set for an attribute you cannot edit the same.
You can opt for either of the below:
1) You will have to create a new attribute with a different name and set the appropriate data type to it.
2) If you wish to use the same attribute name then we will have to delete that attribute from the backend. On the panel frontend, we don't have the option to delete the attribute. Therefore, we will have to delete the attribute from the backend and it takes 24 hours for the deletion process to complete. After 24 hours you can create the attribute with the same name as earlier and set the desired datatype.