LibreOffice enables you to track changes, accept and reject them. This is used when two people are working on a document and want to easily see what the other person changed. Sometimes you want to accept all changes to clear all the underlined text.
To accept all changes in LibreOffice, click the following menu option:
Edit -> Track Changes -> Accept All
This was useful in my case because I was reading an entire document for the first time which had changes underlined from the author because they edited a previous version of the document (which I hadn’t read).