It sounds like you haven't used Styles. In that case I'd do it such:

Create a Style called Swap. For example it could be Webdings just so it stands out for now.

Using the name Gallery and in the Used Font section select the name of the font you wish to change. This will be highlighted.

In the Style dropdown list select 'Swap'.

Repeat for every page. It only reqires two clicks, select font name, change style to Swap.

You now have created a style over your 100+pages and you can set it to Times New Roman, adjust size colour, line spacing etc or whatever you want