This happened to me, and this is what I did.
I let the agent know that I had other agents currently reading the MS, and it was only professional to let them know I had an offer so they could either offer or pass. The offering agent was okay with this, and even expected it.
I contacted the other agents with my MS. I told them I had an offer, but I was still interested in their agency. I wanted to get back to the offering agent before too long, so was X date enough time to review the MS or did they need more time? I gave them two weeks, and they all seemed fine with that.
Then I waited. I thought up what I wanted to ask the agents if I got multiple offers, and I tried to narrow down my choices so I'd be better prepared. If it was between X and Y, who would I go with? kinda thing. Weighed the pros and cons. Oh, I also did this before I contacted the agents, because I had a few that I wouldn't have chosen over the offering agent (the offering agent was a better fit, though all were good agents), so it seemed silly to waste their time reading my MS if I knew I wouldn't go with them anyway.
I got two more offers. I spoke to each agent on the phone and asked my questions, heard their vision for my book, got a feel for them. Then I said I wanted to sleep on it, made my decision, and notified all the agents.
I ended up going with one of the other agents. I felt bad about turning the others down, especially first one, but this is a business decision so you have to do what's best for you and your career.
Congrats and best of luck!