Your problem is in the bolded section.

UPDATE @EntityDateTimes

SET EventID=(SELECT TOP 1 EventID

FROM tkEntityHistory )

WHERE EntityID=@EntityDateTimes.EntityID

AND [DateTime]=@EntityDateTimes.[DateTime])

Because @EntityDateTimes is not in the FROM clause the Query Processor doesn't know what you mean by @EntityDateTimes.EntityID.

What are you trying to accomplish in this section of code? I thinky you may want the WHERE clause to be part of the subquery, then you would need to move the closing parenthesis to the end of the WHERE clause, and on closer inspection it looks like you have an extra parenthesis.

You might try this

[font="Courier New"]UPDATE @EntityDateTimes

   SET EventID=(SELECT TOP 1

                   EventID        

                FROM

                   tkEntityHistory

                WHERE

                   EntityID=@EntityDateTimes.EntityID AND

                   [DateTime]=@EntityDateTimes.[DateTime]) [/font]