I started working as a remote(work-from-home) intern at a very young startup. The internship is for 3 months and I just learned I will have to attend a 3-hour meeting every night. For every meeting missed, a day will be added on to my internship completion date. This will not work out for me. How do I tell them that I won't be continuing with the internship?
What is in your contract?
If there is no written contract, no problem. You just say thank you but no, thank you, that's not the deal we had, and walk away. Stress the part about meetings and completion date changes certainly not being what you agreed to. You have no obligation to respect deal you didn't make.
If the contract you signed does not say a thing about these meetings, you should say something like:
I'm sorry to hear about the night meetings. My contract does not include such obligation and does not give you right to change completion date, and I do not agree to such changes. We may proceed with my internship as contracted, or you may decide to break our contract. If you want to withdraw from the contract, we may negotiate that.
Be sure that you will have it clear and in writing that it is their breach of contract / withdrawal, not anything that can be considered your fault. If you needed this internship as part of your education, include someone from your school in the negotiations.
If this rule was in your contract, you need a labor law lawyer. It depends on jurisdiction but it probably was illegal. If it is part of your formal education, your school will probably provide you with lawyer help, and even if not - should be notified about the issue.
It's odd to have a completion date for a non-contractual arrangement (there being no compensation, it's hard to view this as a trade).
You can amicably inform them that the position they have kindly offered you is no longer appealing to you, and you will not require access to their systems in future. Obviously, you need to return anything which was lent to you (e.g. laptop).
It's an internship, legally speaking I would imagine they have literally no power to hold you to your contract. You are working voluntarily for them. If you were a paid employee and you walked away at this point you would lose your paycheque but since you're not being paid you will lose nothing.
Send them an email saying:
On reflection I cannot commit to attending a three hour meeting every day and for that reason I no longer wish to pursue this internship. Thank you for this opportunity.
Focus on clear communication.
Before quitting, find out what the meetings are about. The meetings may be training for interns. Three hours of training could be wonderful.
If they are not training, try to find out the purpose of the meetings and why they think it is important they extend the internship if you don't attend. Find out what they want to get out of the arrangement and explain what you want from it.
If they see the internship as free labor, you can politely explain that is not what you are interested in or what you promised and move on. Don't burn any bridges; they are a young company and may have no idea how to handle interns yet. There might be great people working there who you will come across again in the future.
Don't feel any pressure to go back. The deal has to be beneficial for both sides.
"You're asking for too much for me to work without pay. Sorry but the opportunity cost for this is simply too great. I won't be working with you." And then you leave a bad review for the company on glass door. Leaving a bad review is standard for a company adding 3 hours to your work day - almost doubling it if you work 4hrs a day and bumping an 8 hour day to 11 hrs - and causing you to question your employment there. It's not revenge, it's truth.
That should be all it takes.