How do I delete an a A Master Grid for two pages of my 9 page Presentation?

SRenee,

If none of these answer your question, then we will need screen shots, including the Pages panel. We need both the Masters section and Pages section—adjust the divider so we can see both.

Do you really mean "I have created A Master Grid for each page" or do you mean "I  have applied a Master page to each page"?

You wouldn't normally create nine master pages for a nine page document.

Hello, you have to create a new Master (B) with the desired grid and then apply it to the pages to modify.

For example: Page 1 with A Master 2-3-4 with  B Master and 5 to 10 again A Master

I hope that my explanation of French will be understandable

You don't need to delete your A Master.

Just create a new master (from the Pages panel menu, choose New Master). You could call it B Master. While it's the active master page, format it as you like for the two pages.

Drag the thumbnail of A Master onto the thumbnails of pages you want to have that format. Drag the B Master thumbnail onto the pages you want with that format.