If you have defined styles, you can change the font in the style definition.  But if you haven’t used styles try the following.

If you’ve used a certain font you can do an advanced search and replace. In the following document, most of the text is Calibri 11 pts. Some sentences are in Times New Roman 12 pts. (I’ve highlighted these in red.)

Click on Replace in the Home tab and place your cursor in the Find What box. Click on More… to expand the Find and Replace dialog box and then select Font from the drop-down Format option.

When the Find Font dialog box opens, select the Font you want to change. In this case I’ve selected Times New Roman and 12 for the size.

Click OK. You’ll notice that there is nothing in the Find what: box, but beneath it is the format you’re searching for.

Now place your cursor in the Replace with: box and select Font from the Format drop-down again.

When you click OK, the dialog box should look like this:

Click Replace All. Word will replace all of the text that was formatted with Times New Roman 12 to text that is formatted with Times New Roman 9.