Use mid statement to split the string at the selstart point. Then, add your new string, and then add the rest of the string, after selstart+sellenght.
text1.text=mid(text1.text, 1, text1.selstart-1) & _
myNewStringToAdd & _
mid(text1.text, text1.selstart + text1.sellength)
Where text1 is the textbox, and myNewStringToAdd is the bit you want to put into the string instead of the highlighted text.
BB