You can simply use WORD's search + replace function to replace all occurrences of a given term in the entire document as you would do with a normal WORD document (thus without opening/closing the corresponding segments).

However, and this is most important, at the end of your work you must *imperatively* do a final cleanup with "Update TM") in order to transfer these changes to your TM. If you forget to do this, the changes you made using Replace will not be reflected in your TM because you did not make the changes by opening/closing + saving the segments.

If the changes only concern the target text (which would usually be the case), I advise to hide the source text before you launch the replace process.