Click on Replace in the Editing group.

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 the incorrect spelling magnitide is replaced with magnitude.

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.