I presume you mean when using Word.
The Global Template (filename normal.dot) is the layout in Word for a blank page. It predefines the margins, fonts, rulers, tabs, headers, etc.
You almost certainly do not want to change it - if you do your "blank sheet" will change in some way.
If you do change it, and want to get the original back - search the computer for the file normal.dot - then change all the filenames it finds to normal.old - then when you start Word again, it will make a fresh template to the original specifications.