Elizabeth,
You can modify the smart campaign at any time, you just change the wait step in the flow. Something to note, once changed from "15 days" to "14 days", anyone in the flow on their last wait day (15th day) will still wait the original 15 days. But anyone who has not waited 14 days already will now only have to wait 14 days.
Also is this email being sent to the same leads every 15 days? Because Marketo, by default, can only send the same email once to each lead to combat accidental sends.
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