You'll need this form:
http://www1.landregis...ary/documents/TR1.pdf
together with these notes:
http://www1.landregis...uments/TR1_notes.html
You can complete those forms yourself but you'll also need to provide proof of identity. That means completing another form (ask the Land Registry for advice) and submitting a certified copy of the photo page of your passport. You'll need to get a solicitor (or other relevant person) to certify the copy. Some solicitors might charge quite highly for that service, a year or two ago, but I got a local solicitor to provide a certified copy of my passport for only a fiver.
In addition to the small sum you'd need to pay to get proof of identity, the Land Registry will charge you £50 for changing the entry in the electronic register.
All of the foregoing assumes that the land is already registered with the Land Registry. If not, you'd need to complete a different form, applying for first registration. The Land Registry can advise you.
Chris