Now what? Keep writing?

Congratulations to you. That's an accomplishment. Celebrate. I am a newbie,
but I would think you would want to wait to hear from the editor to see what
there thoughts are.

First of all, congratulations on finding an agent.

Though I only write fiction, I would say start thinking about another project. It's good to keep writing, but I'd wait to see what any editors had to say about your first book.

This way you're not sitting about doing nothing (not that you would be anyway) but at the same time you're not making changes to your manuscript you may very well be asked to change back at a later date.

I'd keep writing. The time between getting my agent and submitting to editors was months. I fiddled with a few chapters and made some changes that I was glad to have made before I was on a publisher's deadline. They gave me 4 months to write the book and if I hadn't had time to play with it before it was submitted to publishers, I probably would have been very muddled.

If your agent wants a rewrite of the proposal or sample chapters, that might also keep you busy.