You will be able to renew the certificate during 30 days prior to its expiration date. Also, you can do it earlier if you purchase the renewal certificate during domain renewal, please refer to this guide.
Kindly note that the renewal requires not only paying for adding more years to the certificate validity period.
Once you pay for the renewal certificate, you will need to get a renewal CSR code from your hosting company in order to activate it. Activation of a renewal certificate is pretty much the same as the original activation of the certificate.
Detailed instructions on the renewal of an SSL certificate can be found in the How to renew an SSL certificate article.
The price for the renewal SSL certificate is, as a rule, a bit higher than for the new purchase. Feel free to check the pricing here.