changing diff oil, can someone please help me out

[blueb8llz]

i have never seen the inside of a diff besides the pictures on the instruction manuel. for the last hour or so, ive been just reading up on diffs etc etc...
so i want to change my diff oil in both the front and back on my emaxx 3905
i hear i want heavier in the front. i dont do any racing at all, just bashing. im thinking 100k in the front and 75k in the back. what do you guys suggest?
but my main concern is this, where do i put the diff oil? i have no idea or clue. i hear people pack the cup with lube. i cant find where people put the new oil. all i can find is, im suppose to clean with denat. alcohol...but is that for the regreasing and lubing...im sooo confused with all this. can someone please shed some 411