From your Discord Bot project's dashboard, use the Start Bot / Stop Bot control to bring it online or take it offline in Discord.
When your changes actually take effect
Edits you make to commands or events while the bot is already running are saved immediately, but only take effect the next time the bot is started, not instantly mid-session. If you've just changed a command's behavior and it's not reflecting your edit in Discord, stop and start the bot again to pick it up.
If the bot won't come online
See "My Discord bot won't come online" in Troubleshooting, this is almost always a token or Developer Portal settings issue, not a problem with your project's logic.