Based on your screenshot, I can tell you are on a older version of Forms.  I can't remember specifically what version changed, I think around 10.3 or 10.4 - or maybe not until 11 - but there would be a search field and an Edit button there are the top of the variable list.  That's the easiest way to edit a variable, using that Edit button.

I delayed upgrading past 10.2 for ages for fear of broken scripts and other issues (and some of that fear proved to be valid in the end), but now that I have upgraded (I'm now on Version 11 Update 2), I am so very happy that I did and this functionality is one of my top reasons why.  If you've been considering upgrading, this is a good reason why you may want to make that leap.

Prior to that functionality being added, the only way I know to edit a variable value on an in progress instance (other than with task in the instance) was directly in the database.  I did this a lot when we were on version 10.2, but you should proceed with extreme caution.  I know Laserfiche would likely tell you not to proceed without support.  You shouldn't attempt any edits to the database unless you are very knowledgeable with the database - and even then, you probably shouldn't.

I'll say this...  If you'd like to examine the variables in the database, here's some info on how to find them relatively easily.  Go back to the History Tab and find the "submission id" - specifically you need the one from the task where that field was populated.  In your case, since your screenshot shows blank values - you might need to start with a different instance where the values were populated to make it easier to identify the fields you are looking for and then come back to this instance/submission armed with that knowledge.  Run a query like this (replacing the 999999 with your submission_id): 

SELECT *
FROM [LFForms].[dbo].[cf_bp_data]
WHERE [submission_id] = 999999

This will return a list of all the field values that were populated on that submission.  If the values of the fields you are looking for are unique, you can probably determine what "attribute_id" applies to the indicated fields without knowing which field is which - otherwise you're going to need to dig through several other tables to determine which attribute_id belongs to which field.  In my case, I was usually looking for something with a unique value, so I was able to determine it just from this one query - but it's going to depend on your instance and submission to dictate that possibility.  In any case, once you know which attribute_id belongs to the field you are looking for, you can modify the query to see that particular field for that particular submission (replacing the 999999 with your submission_id and the 55555 with your attribute_id): 

SELECT *
FROM [LFForms].[dbo].[cf_bp_data]
WHERE [submission_id] = 999999
  AND [attribute_id] = 55555