How does a pitcher earn a win in baseball?

TO UPDATE THE PERSON'S POST ABOVE: MLB Rules state that a condition for a "save" is that the pitcher is not eligible for a "win". As such, a relief pitcher entering the game prior to the 5th as a reliever would get credit for the win, with no save allocated. If a pitcher pitches 6 innings and leaves the mound with the lead to field another position, then returns in the 9th to close, he remains credited with the win (if his score holds) with no save allocated.

As for a team taking the lead with a pinch-hitter for a pitcher, the actual change of pitcher of record does not take effect until the next pitcher "takes the mound". Though the pitcher is removed for a pinch-hitter, the performance of the team will affect his ability to win the game. In other Words, if the pitcher leaves the game while his team is on offense with the lead, he can only earn a win or a no decision. If he leaves the game while his team is on offense with a tie or behind, he can win, lose, or have a no-decision, depending on what the score is when the next pitcher takes the mound.