Normally your domains are set to auto-renew when you register a name with us. This is to ensure that you do not lose your domain name by forgeting to renew it.
Sometimes, you might want to renew it manually ahead of time. This is how you do it.
Login your your billing portal as usual
Once logged in, click on the square that says Domains:
Now, click on the three dots next to the domain that you want to renew and click on renew: