An improver was a landscape architect/designer who made changes to the layout of country houses and their surroundings, a fashionable activity in 18th century and early 19th century England.
Amongst the most famous landscape designers of the period were Lancelot 'Capability' Brown (1716-1785) and Humphry Repton (1752-1818). Both produced numerous designs for the grounds of English country houses. Brown worked on Alnwick Castle (Northumberland), Chatsworth (Derbyshire), Petworth House (Sussex) and Wimpole Hall (Cambridgeshire); Repton worked on Kenwood House (London), Plas Newydd (Anglesey), Woburn Abbey (Bedfordshire) and Stoneleigh Abbey (Warwickshire). The latter was for 400 years the country seat of the Leighs, relatives of Jane Austen.
It is Humphry Repton who is later recommended to Mr Rushmere by Miss Bertram when he is considering the possibility of 'improving' Sotherton Court.