Pet Violation

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New to being a landlord. My wife and I just purchased three duplexes in the state of Missouri for investments. All units were occupied when purchased with leases.
Upon taking possession, one renter, who had only been there one month under old management, decided that she could not live without her dog. It specifically states 'No Pets' in lease. What are my first steps? What type of documentation should I give these tenants to make life easier down the road? And were online can I get a copy? Someone mentioned that I need to serve them a "Notice to Perform", is this correct? Thx for helping.