I have two fields. When an item is selected from field 'A' and autofill calc is applied to field 'B' but when I go back and change 'A' , field 'B' remains the same. - Help

That's how the auto-fill option works. Once the field gets auto-filled, the value won't change. You have to manually delete the value for a new one to auto-fill.

Try changing the field type to Calculation. The value of a calculation field can change when any of it's constituent fields change.

Or create a button to recalculate as necessary.

You can also create a calc field to check if the autofill field is "correct", and alert you if it isn't (even use this field as the button for the script). That way, you can override the auto-entry if you want, and also reset it as necessary.

So many options, so few clients.