Method 1:

1. From Shopify admin, go to Settings > Checkout and accounts
2. Scroll down to the Checkout Language section.
3. Click “Manage checkout language”.
4. Scroll down to “Checkout contact”.
5. Edit both Address2 label fields to add “Suburb”.
6. Click Save.

Method 2:

1. From Shopify admin, go to Online Store > Themes.
2. For your published theme, click Actions > Edit Languages.
3. Search for “Checkout & system” in the menu. 
4. Scroll down to “Checkout contact”.
5. Edit both Address2 label fields to add “Suburb”.
6. Click Save.