to delete attribute from a model open the Object Viewer Window | Object Viewer...) and try to find the attribute by name:
Re changing data type: there are various ways how to achieve the desired result. In case your model is larger and criteria for the change cannot be to set easily, I recommend export/import to Excel.
Open your model and select File | Export | Export to Excel...
The model metadata will be exported to excel. Change the data types on sheet Attributes, save the file and close Excel.
Then import the Excel file back to your model.