I agree with Neil. Also, sometimes it’s just better to leave the ums in. It’s natural.
One other thing to try is to grab a few frames’ worth of room tone and lay it over the um without cutting the video. If the lip flap is not too great, it reads on screen as just a silent pause.
By mixing all the different techniques Neil mentioned and not using any particular one over-much, you will get a better overall product.
The REAL cure for this, however, is in using an experienced cameraman to change up his framing live, in those split-seconds, when the long pauses happen, and a field interviewer that isn’t afraid to ask the person to say it again a second or third time, perhaps in a different way. THAT is what gives you enough material to cut around a lot of mistakes and pauses.