Change segments status in SDL Trados

Select all the segments where you want to change the status.
To do this, place the cursor in the first segment that you want to change back to untranslated. Make sure it's in the segment number column on the left.  Now go the end of the file (ctrl+end) and click shift+left mouse click in the last segment number.  With all the segments selected, do a right mouse click and select "change segment status" to "not translated"