need to convert 10^9/L to 10^3/ul, it will apply on complete blood count?

To start, and maybe this is why you are confused, you have that backward. You cannot possibly convert 10^9 L to 10^3 uL, however, 10^3 L = 10^9 uL.

You do this using "dimensional analysis". Start with your starting number, which we will say is 10^3 L. You also need the conversion factor between liters and microliters which is 1 L = 10^6 uL.

10^3 L x [(10^6 uL) / (1 L)] = 10^9 uL

The statement in the middle of the above expression is the conversion factor, and just equals 1, but it is how you switch the L in the beginning to uL, and cancel out the original L.

ADDENDUM 18 Jun 12 1000hrs

I will try to make myself more clear. You start with 10^3 L. There are 10^6 uL in 1 L. You need to multiply your starting amount in L by 10^6 to turn it into uL.

10^3 L x 10^6 = 10^9 uL

The way I explained it previously is the proper way to do it. You multiply your starting amount by the "conversion factor" which is a statement that is equal to 1, but allows you to change units. Since there are 10^6 uL in 1 L, you can say that [10^6 uL / 1 L] = 1, since anything divided by itself is 1.