static class VS interface

Recently I found out that the Singleton design pattern isn't recommended for use by many experts. same about static classes.

I wish to consult with you about the best solution for my c# program. I have some static classes, for the general functions. for example - a static class named "SearchTools(.cs)", which keeps the BL functions of search, "NavigationTools" and so on. all these classes are static.

In the most basic form:

public static class SearchTools{
public static Function1();
public static Function2();
private static Function3();
}

What is the best way to redesign my program, in order to achieve single responsibility and reduce coupling, by using interfaces?