Toniechang,
I think your standard, everyday, run in the mill Windows functionality will work (cut/copy/paste...). Double click the existing text that you want to copy so that your annotation editor pops up. Double click on the text in the annotation editor window to highlight the word(s) or otherwise highlight the text you want to copy. Press <ctrl-c> to copy the highlighted text to your clipboard (<ctrl-x will erase the highlighted text while adding it to the clipboard btw). Then just click the text you want to alter, highlighting the part or parts desired, and press <ctrl-v> to copy the clipboard contents over the existing text, effectively replacing it.
<ctrl-x> = cut
<ctrl-c> = copy
<ctrl-v> = paste
There are a number of "hot-keys" imbedded into UG that you may want to get up to speed on, as they really are time savers and helps to reduce muscle fatigue in your fingers. Online docs should help... ;-D
You can use edit text then delete the text you don't want and hit the button to place text. You can then place the text. The next time the annotation editor shows it will show the copied text.
