First things first, this is for if you absolutely NEED to cancel or reschedule. These are not instructions for what to do when you are 20 minutes out from the game and just don't feel like running.
We have all been there. You have a game planned and then something comes up that drastically changes your schedule in an instant.
The most important thing to do if you need to cancel or reschedule a game is to get in touch with your players. Give them as much warning as you can, and try to find a new time that will work for everyone. Get in contact any way you can. Send a message through the site, an email, group chat, discord, anything that you have that will let you get a hold of them, use it. The last thing we want is a GM to have an emergency that they need to move a game, and the players don't know that resulting in them being upset with the GM. Communication is key.
Once you have talked with your players and found a new time that works for everyone, you can reschedule the game from your adventures. Just find the game you are trying to reschedule, and click on the pencil next to the currently scheduled game.
That will bring up a screen where you can cancel or reschedule that session.
Once you have finished setting up the new time, make sure to thank your players for understanding, and when you get them back around the table, make sure you give them a great game!