Update int column in table with unique incrementing values

declare @i int  = (SELECT ISNULL(MAX(interfaceID),0) + 1 FROM prices)


update prices
set interfaceID  = @i , @i = @i + 1
where interfaceID is null

should do the work

DECLARE @IncrementValue int
SET @IncrementValue = 0 
UPDATE Samples SET qty = @IncrementValue,@IncrementValue=@IncrementValue+1