CREATE TABLE test (street varchar(255), flat varchar(255));
INSERT INTO test VALUES ('ЧЕХОВА', '31 Д 81');
SELECT street, flat, street + ' ' + SUBSTRING(flat, PATINDEX('%Д%', flat), LEN(flat)) AS new_street, SUBSTRING(flat, 0, PATINDEX('%Д%', flat)) AS new_flat
FROM test
UPDATE test
SET street = street + ' ' + SUBSTRING(flat, PATINDEX('%Д%', flat), LEN(flat))
,flat = SUBSTRING(flat, 0, PATINDEX('%Д%', flat))
SELECT street, flat
FROM test