That depends where the echo is coming from. If you are using a laptop with the default speakers and built-in microphone, chances are good that the audio being played over the speakers is being picked up by the mic. The fix would be to wear headphones, so there is no audio for the mic to pick up, or get a lavalier mic so it is further from the speakers/closer to your face and can be run at a lower volume, so any speaker audio it picks up will be much quieter/inaudible.
It's also possible if you're using a separate webcam that both the laptop mic and webcam mic are both active; you'd mute one or the other to fix this.
As far as completed recordings, unless you recorded your mic and speakers to separate audio tracks for later re-mastering, there's nothing that can be done to remove the echo; it's baked into the audio track now.
If you have separate audio tracks, you can use the old mastering technique of flipping the speaker audio phase and applying it to the mic track to try to cancel itself out, then put out a normal-phase speaker and the cancelled mic track as your final output. But that's outside the scope of OBS support, you'd need to work with whatever DAW you're using.
