There are several ways of removing the negative effects of karma and these include meditation, purification, avoiding negative thoughts and by gaining wisdom, according to the View on Buddhism website. Karma simply means "action" in Sanskrit. Buddhism takes this definition a step further calling it as the motivation and intention behind the action, according to the words of Buddha himself.
Good and bad things that occur are not exactly karma, but are based on it, being the effect or result of an earlier action. Therefore, what an individual is experiencing at present, whether positive or negative, is a result of karma. The four laws of karma also state that its results are equal to the cause, there are no results without a cause, once an action is done the result is never lost and karma expands.
Meditation is the first step in removing the negative effects of karma, particularly the Sanchita karma and all other means of removing the negative effects of karma requires meditation. It is also the means best taken to remove negative thoughts, thus lessening the chances of committing negative actions in the future. Purification also requires meditation contemplating on regret, on the sentient beings that were affected by one's negative actions and on one's commitment to doing only positive actions. Finally, meditation leads to wisdom and emptiness when all types of karma no longer have a hold on the ego.