How should i tell my boyfriend that I’ve cheated ?

No need to tell. Just forget and start over again with him. Life must go on. We are humans we make mistakes sometimes

You should first ask yourself what you wish to achieve by telling him, particularly if you had an "accident" and the only way he would know is that you just tell him. Instead of just you feeling remorse (maybe), you would be inflicting him with pain as well. If otherwise there are good chances he can find out, then you can choose to tell him. The best time to do this is when he is in a receptive mood and not affected by another issue. You can then preface what you want to say by a contrite expression ("I messed up", for instance), say "I had sex with someone else" and then let him ask you questions. It is best to let him take the lead in that conversation.