How to replace all occurrences of a string with a specified string from rich text box in c#?

Respected Madam, being more succinct, just look at this screenshot illustrating the objective i want to achieve:

The code i am working with is as follows:

When user clicks on "Change All", this code runs to achieve the objective:

private void button5_Click(object sender, EventArgs e)
        {
            
            String selectedWord = this.suggest_list.SelectedItem.ToString();
            String word123 = this.NIDict_list.SelectedItem.ToString();
            String abc = Regex.Replace(this.TextContent, word123, selectedWord);
            this.FirstForm = new spellChecker_frm(abc); 
// i am throwing the result to the first form into                                                                                                                                                                                      //TextSpelling form. I just want that //as i am still working in TextSpelling Form so what should be done to reflect the result in rich text //box without closing up the TextSpelling form.

//the rich text box which is behind 

//TextSpelling form. I just want that //as i am still working in TextSpelling Form so what should be done to reflect the result in rich text //box without closing up the TextSpelling form.

        }

The code for the Form1 where rich text box exists:

 public spellChecker_frm() // Default constructor called when program starts.
        {
            InitializeComponent();
            i = 0 ;
        }
        public spellChecker_frm(String content)// overloaded constructor i made.
        {
            InitializeComponent();
            this.textContainer_rtb.Text = content; // content thrown from TextSpelling Form using avove code.
        }

Now this overloaded constructor is expected to again fill the rich text box with the content being thrown from TextSpelling form but i don't know why not it doing that because i reckon that the form 1 is not being called again?

Secondly, i ain't getting the 3rd option as it seem to me a bit alien because it s incomplete as i want a full fledged example.