Actions are being phased out in June (much to our dismay). So I would attempt this as a pipeline.
However, if you are going to try the action - actions can only edit records in a report link. And an action will edit ALL records in that report link. So for example, if you have a child table, when your action takes place, you can select edit records in 'this child table', and then on the bottom select which fields will be edited and what value they will get. If a field is not available, that means it is not an editable field.
If you are trying to edit a field in the record where the action is taking place, you can sneakly do it with an action by creating a report link that shows itself, though that is not best practice. In that case you should use a webhook (also being phased out), or the real 'new' way is with pipelines.