I know nothing about the song. But, in general, the phrase "round up," in English has at least a couple of meanings, the first of which is likely to be the one that applies to the song: 1) To gather a group of people (or sometimes things) together. 2) To record the answer to a sum that ends in a half (such as 39.5) as the whole number immediately above the actual answer to the sum - so in this case the actual answer to the sum is 39.5 but, if ""rounded up,"" the answer would be written down as 40.