EDIT
For MySql it'll be
UPDATE table1 t1 INNER JOIN
table2 t2 ON t2.id = t1.id
SET t1.value = t2.value
sqlfiddle
Original answer was for SQL Server
UPDATE table1
SET table1.value = table2.value
FROM table1 INNER JOIN
table2 ON table2.id=table1.id
sqlfiddle