To make the apology mean more to your child, encourage actions in the form of gestures, rather than just using words to apologise. Giving the affected person a hug or shaking their hand is a more tangible way for your child to say “sorry”. If your child has done something like deliberately tip over their toy box as an angry response, get them to help tidy up as part of their apology.
If you teach your child to apologise using the correct words and repeat the format often, they will learn it. If your child is fighting with a friend or playmate, initiate an apology by suggesting the appropriate verbal response, or course of action. Make sure that it comes from your child, not from you directly. Always praise your child for apologising on their own, or for admitting that they have done wrong.