I am having trouble working out exactly what you are asking but see below it may help you?

UPDATE

--table to update

Values1

SET

--column you want the value inserted into and from

Values1._Value = Values2._Value

-- normal join

FROM

Values1

INNER JOIN

Values2

ON

Values1._ID = Values2._ID

-- use where as your if statement

where Values1._Value = Values2._Value_2