How do I change the font in all pages in Pages app at once?

I want to change font everywhere in a pages document in one go without doing it manually for every page.I would like to know how can I do this.

First select all of the document by pressing Command+A. Then click on Format > Style on the Pages toolbar at the top and choose the font you wish on the sidebar on the right.

You select the menu item "Format > Font > Replace Fonts…".
This brings up the font replacement dialog, showing all fonts contained in the document. Here you can replace any of the fonts in the document with any font installed/available on your system via the drop-down menus in the "Replace With" column.