Arithmetic overflow error converting expression to data type int When updating table

Yes, if you try update t1.count1 with t2.count2, and count1 is an int and count2 is a bigint - the logic thing to do is to change the datatype of count1 from int to bigint.

There might be a solution in the case that if the values of count2 did not need to be bigint - i.e. the values falls within an int, maybe you could cast count2 before an update was made.