"ug" stands for microgram. You can convert microgram into grams by multiplying a certain factor. For this case, 1 microgram is known to be 1x10^-6 g. This is the conversion factor that will be multiplied to 3.54.
3.54 ug ( 1x10^-6 g / 1 ug ) = 3.54 x10^-6 g