Desktop:
1. Navigate to https://www.instagram.com/
2. Click 'More' in the bottom-left
3. Select 'Settings' with the gear cog
4. Click the blue 'Switch to professional account' button
App:
1. Open the Instagram app on your mobile device
2. Click the profile picture in the bottom-left corner
3. Click the 3-line menu in the top-right
4. Select 'Settings' > 'Account'
5. Scroll down and select 'Switch to professional account'