You have to say “I am sorry” or “I apologize.”
Whatever you say after this determines how sincere you are; Let’s say you leave your friend stranded at the airport, and you do not call back to find out how they found their way to town.
Do not apologise like this; “Sorry I couldn’t pick you up, I was busy.” This lacks empathy and you are not taking responsibility.
It sounds like you are justifying your actions. Instead say, “I am sorry I couldn’t pick you up at the airport, you must have been stranded. I should have called to tell you I could not make it. I apologize.”
Another bad apology would sound like; “I am sorry I upset you. I didn’t mean to say that, I was in a bad mood.”
This isn’t an apology but a justification for your actions. It means, “Blame my bad mood for what happened and not me.”
Admit that you were responsible
Accept what you did wrong instead of giving reasons trying to escape accountability.
If you said some words that hurt your friend instead of explaining it away, apologize if they found them hurtful, saying things like "I didn't mean it that way." isn't an apology.
Ask to make amends
In a situation when you can do something to fix the situation always offer to do it or ask, “Is there anything I can do to make it up to you?”
Do not make extravagant promises; always do what is within your power.
Improve your behaviour
An apology without changed behaviour is manipulative. Are you saying you are sorry to get out of an uncomfortable situation or would look at your actions and see how you can improve and become better?
Finally, be okay if they want some time and space away from you,