Hostinger's PRO Panel allows you to manage your customers and their websites with ease. You can find it in hPanel → PRO Panel:
Once you're in the PRO Panel, go to Clients:
Next to the preferred client, click on Manage:
You will be redirected to the customer's account that you manage. To go back to your own Hostinger account, click on Go back to my profile in the top-right:
If you haven't invited anyone to work with you yet, take a look here: PRO Panel: How to Add a New Client 💡