A family of Microsoft word processing software products for creating web, email, and print documents.
In Word 2010, you have to click Insert on the status bar to change it to Overtype. You would also need to press Ctrl+U to turn on Underline formatting for the text that you are typing into the underlined space.
Depending upon how the lines were created, the underlined for text that you type may not line up exactly with the original line.
This is getting closer. What I found was that Microsoft decided to disable the insert key by default. To fix it once and forget it forever, go into “Word Options” >>”Advanced” Tab >>”Editing Options” header >> put a check mark in the box “Use the Insert Key to control overtype mode”.
Thanks to Yves Dhondt for his response to a similar question back in May 2010
If the lines are important you'll need to know they were created. If they just repeated underscore, I found it easiest to delete their "underscores, enter my information, then use the "underline" function to put them back