You cannot edit dimension text using the standard Edit => Text command. All changes to dimensions will need to be done under the Manufacturing => Dimension Environment. Once in that mode, the right click context menu contains the operations you can do to dimensions, Change Text being one of those options. Once you select CHange Text and select thedimension text you want to change, the Options tab has areas to change the value and text. There is a little "?" beside the text box that will show you different special character strings to add new lines, etc. to the existing text.
I will have to say though, it is not very intuitive.
Hope this helps.