I need to know how we can unit test asp.net web services web methods having connection string and which is modifying the data base

I have a asp.net web services service layer which has [WebMethod]s. Those web methods have connection strings and are modifying dev db. There is no separate data access layer. WebMethods are modifying the db through stored procedure. I want to unit test those web methods using nunit. But could not figure out a way to do it without modifying the db. I am willing to modify my service layer so as to completely separate the data access layer and then unit test it. Any leads towards good practices to do these things will be appreciated.