Not sure what you mean by heads up in a cash game.
Everyone folds except you and one other player.... just play normal.
Table is empty except you and one other player... I play loose/passive.
The blinds aren't changeing. Since it is just you and one other the
odds are better when you get a face card or connectors.
Tourney I assume it is final table heads up... play loose agressive. For the reason stated above plus sense of urgency due to blind increasing. SB I call/fold while BB I call/raise. Depends on my cards and how the other player plays.