If you purchased a domain at Hostinger, you can easily renew it on the Billing page. Just click on Billing at the top menu and then Renew Now in front of the domain which you wish to renew:
This will redirect you to the cart, from where you will be able to renew your domain.
Domain renewal is possible only 2 months before the expiration date or less (based on the TLD)
Some TLD accept renewals for longer than 1 year. If you want to renew your domain for a longer period, please contact the Customer Success team to know if it’s possible for your domain
You can renew your domain and other services automatically, you only need to enable an Auto Renew option
If your domain is registered elsewhere, you should transfer your domain to Hostinger to renew it here. Alternatively, you should renew it at the current domain registrar
Detailed explanation about the domain life cycle