If you forget there is a possibility that you could face a fine of up to £1,000. However, experience tells us that if you rectify this as soon as possible then you may be excused having to pay a fine.
It is quite simple really; just go to https://www.gov.uk/renew-driving-licence and you will be able to do it in just a few minutes. If your passport is in date they will use the photograph that is on there, but if it isn't, then you will need to apply for a new one using forms from the post office - http://www.postoffice.co.uk/uk-driving-licence-photocard-renewal-faqs
You should get a reminder (D798 renewal notification ) around two months (56 days) prior to the date or renewal, which you take to the post office, where you can also get your new photograph and the process is quite simple, providing you take with you the relevant documentation (and money).
If you have the old paper driving licence, you will not need to exchange it until the day before your 70th birthday. If you need to renew a vocational licence, there are different rules as to the age of renewals all of which can be found on the government website at http://gov.uk.
If you are interested in driving lessons including advanced lessons or trailer lessons, then how about contacting Toad's School of Driving in Cambridge