I take that back!
Having re-read the original post, I see that you want 5 to be multiplied by 0.9, so Bozza was right. However, if you use INT, a 5 in cell A1 will return 4 in B1; if you use ROUND, it will return 5.
So it depends what you actually mean by "B1 to be an integer".
My final...
If the number was 5, it would still return 5 as the answer. If it was 5.1, it would multiply by .9, if it was 4.9 it would return 4 (integer) or 5 (rounded)
Can the values in A1 be fractions?
If so:
Int(if(a1>5,a1*0.9,a1))
If by integer, you just mean a whole number, this will give slightly different results for fractions:
Round(if(a1>5,a1*0.9,a1),0)