Slices come from putting a spin on the ball from left to right (right handed golfer). The spin on the ball is caused by the path of the club through impact. Slice spin comes from an outside to in swing path. Many techniques are taught on how to fix a slice. Start with your setup.
The most common mistake starts in the setup. If your feet and shoulders are open to the target (left of target for right handed golfers) your swing will likely follow. Make sure your feet and shoulders are in line with the target or even closed to the target.
Then try standing just slightly farther away from the ball and think about hitting the inside (closest to your body) half of the ball.
Lastly, try placing a towel between your right elbow and right rib area (right handed golfers). When you swing, make sure the towel doesn’t fall to the ground. This will help keep your right arm close to your body and stop you from casting the club away from your body and hitting the ball with left to right spin.