Simply do good deeds not evil ones and your alignment will recover.
OR
Start another profile and play as good from the start. In this way you can play how you feel.
I tend to have too many profiles but having several helps depending on what you want to do at any time, Play good evil MP etc..
While you can change your mind on how you want to play it makes it hard work changing creature actions and alignment part way through a game. Although this can be a killer tactic for some playing styles having opposite alignment God and Creature, it needs a lot of skill to maintain this though and is best left alone until you have completed the game 2 or more times.