You will not lose any extra time. The chosen renewal period will be added on to the end of your current remaining registration period.
For example: if your domain expiration date is 01/05/2015 and you choose to renew the domain for a year, your new domain expiration date will be 01/05/2016.
Domains can be renewed at any point in time and it can be beneficial to renew early. It can ensure that you are protected against 'dropcatchers' and that there is no downtime if your domain expires. Domains can be renewed for 10 years and there are significant discounts available when you renew for multiple years.
To renew your domain manually, you just need to visit your client area. Go to My Domains, click manage on the domain you wish to renew and then chose the Renew Domain option.
By default, domains are set to automatically renew. They will do so if the payment type is set to credit/debit card and you have valid card details stored on file.