Install the Supabase client in your app. If you're using npm, run:
```sh
npm install @supabase/supabase-js
```

Initialize the Supabase client:
```javascript
import { createClient } from '@supabase/supabase-js';

const supabaseUrl = 'YOUR_SUPABASE_URL';
const supabaseKey = 'YOUR_SUPABASE_ANON_KEY';
const supabase = createClient(supabaseUrl, supabaseKey);
```

Fetch the user profile based on the logged-in user ID:
```javascript
const user = supabase.auth.user();

if (user) {
const { data: profile, error } = await supabase
.from('profiles')
.select('*')
.eq('user_id', user.id)
.single();
}
```

Display the user profile in your app.

Provide a form in your app for users to update their profile details.

Use Supabase to update the profile:
```javascript
const user = supabase.auth.user();

const updates = {
username: 'New Username',
fullname: 'New Fullname',
avatar_url: 'New Avatar URL',
bio: 'New Bio'
};

const { data, error } = await supabase
.from('profiles')
.update(updates)
.eq('user_id', user.id);
```