Replacing the dimension for something new will break the association to the real measure in the geometry.
So changing a dimension to say two lines by using \n …(extra text), as Gully mentioned, might show the correct value plus a second line but changes in the length will not be seen in a new correct value.
You can restore the association by replacing the value by <> when changing the dimension’s text.
Example: an associated dimension’s text is 500"
You replace this value for a two-liner by adding \n extra text
Now also the value 500"is plain text and won’t follow changes. The value stays 500" when making changes
If you however replace the value for a two-liner <>\n extra text you’ll also see 500" and extra text on line two. But 500" will change correctly when making changes to the geometry.
Or try extra text\n<> to have the value on line two. or extra text\n\n<> to get the value on line 3.
etc.