I know it's a bad habit, but I just can't help it whenever I'm speaking! Especially in any presentation or public speaking. It just feels so natural.
The first step is acknowledging it, which it seems like you have. In the beginning, just force yourself to call it out every time you do it. That's all! Just keep a counter going of how much you say it. Or even acknowledge it out loud, like "oh there's another time I said uhm".
From there, you can try to replace it with a pause, then try to get rid of it altogether!
Uhhhhhmmmm...haha just kidding. This is so hard for a lot of people and it definitely takes practice. When you're in a train of thought, instead of saying um, take a deep breath and pause. See if that works.
I use to also do this a lot! Especially during presentations, I found myself saying "um" and looking down. What helped me was studying my points and making sure I had them down! Taking a deep breath, smiling, and putting confident thoughts in my head assisted in ridding that habit. Hope this helps you!