Once you have responses against a workflow or a form it is no longer possible to make changes as the configuration of the fields needs to stay the same to ensure the consistency and integrity of any data already captured.
However, with both workflows and forms, you can clone an existing one allowing you to then make changes.
To do so, you need to create a new workflow or form of the same type as the one you wish to clone - for example, if your workflow is currently am on-boarding task workflow then the new one should be the same.
Once you have created the new workflow or form, next to the item in the actions column you will see an icon of two sheets of paper on top of each other as below:
When you click the clone button, you will then be brought to a screen where you will see a dropdown which lists all the workflows or forms of the same type as your new one from which you can clone the fields - simply chose the workflow or form you want to clone FROM and click submit and this will then clone all the fields into the new workflow or form.
As this new workflow or form now has no responses you can safely change the layout, add new items, remove items and so on.