Updating or refreshing controls with c#

Question

I've made a winforms project that lets me search and add into an SQL database using SQLite. One form has a TreeView and CheckedBoxList that contain the symptoms or species related to an illness. These controls are filled programmatically with their own public non-static function. Another form lets me edit the database, specifically adding a node to the TreeView or list of symptoms. After editing the database, it activates the function to update the mentioned controls. I hoped it would immediately update the control, but it doesn't.
A few solutions I've tried:
this.Refresh();
this.Invalidate();
this.BeginUpdate(); and
this.EndUpdate();
I've done them separately or all at once; tried it for "this.", "Form1." and "treeView1."; but none managed to update the form or control.

Then I thought 'maybe updating the control on Form1 from Form2 is different from updating itself on Form1', so I added Form1.treeView1.Nodes to make sure it focussed on the correct control. This still did not show the newly added node or checkBoxList Item.

Form1:

using System.Data.SQLite;
// ///
public void UpdateCheckedBoxList(SQLiteConnection conn) {
    SQLiteDataReader sqlite_datareader;
    SQLiteCommand sqlite_cmd = conn.CreateCommand();
    sqlite_cmd.CommandText = "SELECT * FROM Species; ";
    sqlite_datareader = sqlite_cmd.ExecuteReader();

    List<string> myreader = new List<string> { };
    while (sqlite_datareader.Read()) {
        myreader.Add(sqlite_datareader.GetString(1));
    }
    string[] Species = myreader.ToArray();
    checkedListBox1.Items.AddRange(Species);
    this.Refresh();
}

Form2:

SQLiteCommand sqlite_conn = new SQLiteConnection("Data Source=" +
            "C:\\Users\\" + Environment.UserName + "\\Documents\\ddx project\\ddx-database.db; Version = 3; New = True; Compress = True;");
sqlite_conn.Open();
new Form1().UpdateCheckedBoxList(sqlite_conn);

Answer 1

Thanks to dr. null's comment, I managed to update the controls. They mentioned my use of new Form1();, creating a new Form1 instead of trying to update the opened Form1. This is a solution:

Application.OpenForms.OfType<Form1>().First().UpdateComboBox(sqlite_conn);

Application.OpenForms.OfType<FormName>().First().formSpecificFunction(args);