Updating Salesforce Campaign from Marketo

You cannot edit that.

If they respond for a second time, that will not change the fact they responded already.

More importantly, what exactly are you trying to do? Is the Lead responding to a new campaign?

Member First Responded date is system generated on the SFDC side. It is populated based on the first time the Campaign Member Status (configured via Advanced Setup in that specific SFDC campaign) is changed to a value you have designated as "Responded".  

If you have multiple status values which all qualify as 'Responded", the first time that particular lead is changed to any of those values will be when First Responded Date is stamped.  If that field updated every time that status changed, it wouldn't, by definition, really be a "First Responded" date.

The only suggestion I can give in this case, really, is to say that perhaps you need to reconfigure your Campaign Member Statuses and determine what your baseline response should really be.  You could also remove members from the campaign and manually re-add them, which should wipe your original First Responded values.

And if you need to track second or third responses (i.e. when someone goes from "Waitlisted" to "Registered" or "Attended"..something like that) maybe rig up a smartlist on the Marketo side that looks at when Campaign Member Status was updated.  Hope this helps.