How to write a getter and setter for a Dictionary?

How do you define a getter and setter for complex data types such as a dictionary?

public Dictionary<string, string> Users
{
    get
    {
        return m_Users;
    }

    set
    {
        m_Users = value;
    }
}

This returns the entire dictionary? Can you write the setter to look and see if a specific key-value pair exists and then if it doesn't, add it. Else update the current key value pair? For the get, can you return a specific key-value pair instead of the whole dictionary?