This answer is for Photoshop and may or may not fit with (Photoshop) Lightroom. The application specifics were unclear when this answer was posted.
Just change some of the brush settings or dynamics. I don't think there's anything to "release" unless you see a little lock icon next to a dynamic.. click that to unlock the dynamic associated with the lock.
Not really certain what you're asking. The default presets are merely settings, once you change a setting, that preset is no longer active. If you want to save a preset with your new settings, then save a new preset.
And the Brush Tool and Mixer Brush tool are separate tools not merely setting variations.