Nagelsmann, whose sacking in March 2023 led to Tuchel's arrival, was tipped to return to Bayern, but has now extended his contract with Germany until 2026
Argentine Aguero fired City ahead after nine minutes with a neat finish but Swansea hit straight back through Spanish forward Fernando Llorente who blasted the ball home from 12 metres.
Manchester City poured forward in the second half, Raheem Sterling wasting a good opportunity before Aguero coolly converted a 65th-minute penalty following a foul on Kevin de Bruyne.
Sterling scored with a fine finish 13 minutes from time to confirm City's sixth successive league victory of the season.