How do I make a field change to whatever when editing a record when blank, whilst keeping existing fields intact?

[desbest]

How do I make a field change to whatever when editing a record, when it's blank, whilst keeping existing fields intact?

I only want to change blank fields.

I mean by making a new operation.

 

[Anti]

First, you should add empty value as first in Switch:

So, if Switch value wasn't set, it will display empty value. Then, you may use Condition field to set some value if it is empty: