HM Registry Showing Incorrect number of rooms

We bought our leasehold two years ago. I see the HM Registry has our flat as a 1 bedroom 1 bathroom but it is 3 bedrooms 2 bathrooms following the previous owners doing a loft extension. How do I have the number of rooms/bathrooms updated so they reflect correctly on the registry? Thanks. 

Maria -m we don't register the number of rooms etc on the land register 
If the demise (extent) of the lease has altered then this would normally mean a new lease/deed of variation is required between the landlord/tenant as appropriate.