you can use different actions or method..Maybe the simpliest is using a set variable action and set the value using the Replace inline function (the correct syntax is displayed in the form).
You can use a Build String Action, and as jackgelo suggested use the Replace inline function. But because you're dealing with a comma you'll need to wrap that part of the function with a text start/end so that it doesn't break the function. Here is the function you'd need to use.
another, maybe slightly more advanced option, is to use the Regular Expression action. Might be a slight overkill for your requirement but if you need to do replacements based on patterns, the Regular Expression action is definitely the way to go.