Show hidden marks.
Showing non-printing formatting marks in Microsoft Word
Show the ruler.

Chances are good that the paragraph the page number is in is right-justified.
Press Ctrl+L to make it left-justified. Type your text at the beginning then press the tab key or insert an alignment tab to move the page number over into the right side again.

Just a guess. As Paul said, we could tell with a sample document (not a screenshot).
How to attach a screenshot or file in this forum.