There is never a point when speaking where you should say “umm” or “uhhh”. An occasionally “hmmm”, I think, is alright to buy time after someone asks you a question and you need to think of an answer, but a “good question, let me think about that for a second” is an infinitely better response.
You can remove any and every instance of “umm” (and similarly “uhhh” for the rest of this post) from you speech, instead leaving just silence where you would have said it, and your speaking will tremendously improve. People will pay more attention to what you’re saying, you’ll appear more confident/knowledgeable, and you’ll be much more enjoyable to listen to.
You don’t need to constantly be making noise when you are speaking. People will continue to listen if there is a brief pause in your cadence, this actually makes it more interesting to listen to. If you really need to think about what you’re saying, you probably shouldn’t be the one speaking at the moment in a conversation. If you’re presenting, you probably should have practiced more.