To work on individual tracks with a fragment's configuration (e.g. your selection of mutes and pattern numbers for the different tracks) intact, while a fragment is playing, hold value and long-press to switch to any track.
This lets you tweak parts of your song as it is played by a fragment. This mechanic is key to working on full songs, rather than simple loops. Note that this mechanic also works from Live mode, where this can be used to quickly mute different tracks or working on scenes (the fragment "equivalent" of Live mode); you can even go back and forth to different tracks via Live mode and keep any mutes and configurations intact.
Note that stopping playback will reset the pattern numbers across all tracks to a predictable state; the pattern indicated on the screen. E.g. if playback is stopped with the screen indicating "09.bS", all patterns across all tracks - not just the bS track - will reset to pattern 9.