Hi Dianar,
Sorry for the delay in response to your query.
The fillable form you are referring to, are you trying to make changes to the data you have entered earlier or is there any predefined field in which you are trying to make changes?
1- If you have applied signature on the form and saved it then you would not be able to make the changes in the form. After applying signature the form becomes flatten and restricted to make any further changes.
You can refer to the related help discussion here- How to edit fields in PDF form after saving
2- If you wants to make changes to the applied forms filed, then you need to edit the form in Adobe Acrobat DC.
PDF form field properties, Adobe Acrobat .
Hope this would help.
Regards,
Akanchha