If you've added a new field to an Asset Type, you can update your existing Form to include this field in the Asset Item or Asset List.
In the Web Portal go to Forms > Existing Templates > Select the Form > Form Builder > Select the Asset Item or Asset List > Set the Asset Type to None > Refresh the page > Re-select the Asset Type.
This will refresh the available Asset fields to include any newly added fields.
To display the new field in your Form, drag and drop a Text Field into the Asset Item or Asset List. Select the Asset Item or Asset List again and choose the corresponding new field from the Asset Field dropdown. Once completed, Deploy / Publish Template to save the changes.