Change modal alert Squeeze Form Addon

I´m purchased the Squeeze Form Addon for Simple Download Monitor plugin

My problem is when I push the download button within complete the name or email fields a modal boxes appear, like the image attached:
http://www.ciocentropsicologia.com/capture.jpg

I want to customize this modal box, I don´t like this tipe of alert and I need to change this alert for another one, with some type of JQuery or similar, or change any code line in php files.

How I can modify this modal alert?

Thanks so much

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