Click on Edit
Click on the double down arrow at the bottom of the menu
Click on Replace

Type the word you want the programme to find for you in the
Find what: box.

Type the word you with which you want to replace it in the
Replace with: box.

Click on Find Next

In the example above I am replacing the word magnitude with size.

Once the programme finds the word you are looking for, you have the choice of ignoring the replace (by clicking on Find Next again), or you could perform the replace action by clicking on Replace.