How to fix leaks in ceiling?

First you have to find the source of the leak. There is a product called Flex Steel which will seal the leaks on the roof once you find out where the water is coming in from.

Water can be tricky. Check all your flashing, because that is usually the source of the leaks.

Do not use Flex Seal. It's breaks down and deteriorates. Would be a band aid fix. You neet to find the leak. Use a tar based roof repair in caulk tube or can. Spread it under the shingles or around flashing.

First you need to find the point of entry od the leak. Point of entry can be far from where the leak in your ceiling is showing up. You need to understand how water travels through your home. Make sure it is not from a leaky water pipe or toilet. Then you can repair the ceiling. Otherwise you will continually repairing the same thing over and over wasting time, supplies and money.

Start here:

https://www.todayshomeowner.com/video/tips-for-locating-hard-to-find-roof-leaks/

Find the cause of leaks, water pipes, showers above or from roof. May need to tear out part of ceiling to see true situation

I din't think there is one answer. Is it a single story house with an attic? Is it a two story house and the leak is on the bottom floor ceiling?

If is is a single story and you can get in the attic you should be able to find the leak even if you have to have somebody spraying water on the roof in the area of the leak inside.

Fixing the leak is the easy part, finding it is the harder part.

If it is coming from a second story I would assume it's coming from a bathroom and that should not be hard to find a leak from a water line or toilet.