Head over to the Firebase Console. Pick the project where your Realtime Database lives.

Click on the "Database" section in the left-hand sidebar. This will open up the Realtime Database for your project.

Click on the "Rules" tab. You'll see the current security rules for your Realtime Database.

In the rules editor, you'll notice the rules are written in JSON. The basic structure looks like this:

{
  "rules": {
    ".read": "condition",
    ".write": "condition"
  }
}

To make your database publicly readable and writable, change the rules to:

{
  "rules": {
    ".read": "true",
    ".write": "true"
  }
}

Heads up: This setting isn't recommended for production because it makes your database open to everyone.

To limit read and write access to authenticated users, set the rules to:

{
  "rules": {
    ".read": "auth != null",
    ".write": "auth != null"
  }
}

To set access rules for specific nodes, customize the rules within the database nodes. For instance, to allow everyone to read but only authenticated users to write:

{
  "rules": {
    "publicData": {
      ".read": "true",
      ".write": "auth != null"
    },
    "userData": {
      ".read": "auth != null",
      ".write": "auth != null"
    }
  }
}

You can create more complex conditions using Firebase Realtime Database variables like auth, data, and newData. For example, to let users write only to their own data node:

{
  "rules": {
    "userData": {
      "$uid": {
        ".read": "$uid === auth.uid",
        ".write": "$uid === auth.uid"
      }
    }
  }
}

Once you've set the rules you want, click on the "Publish" button to save and apply the changes to your Realtime Database.

Testing is super important to make sure your rules are correct. Use the Firebase Emulator Suite or try accessing the database from your app to see if the rules work as expected.

Keep an eye on access logs and usage to make sure the rules are providing the security and functionality you need. Adjust them as your app evolves.