On Mac, there is a system feature that allows you to replace straight quotation marks in your text. To do so, open the sheet and go to Edit › Substitutions › Show Substitutions. Then, make sure Smart Quotes is checked, select the quotation marks of your choice, and click Replace All.

Alternatively, you can also select certain parts of the text and click on Replace in Selection to replace them only in the specified part of the text.
This feature is currently unavailable for iPad and iPhone.