You cant change it, mainly because there absolutely no need to. Pci-e 2.0 cards work fine in 3.0 slots and vice versa.
can you change it for the new versions? my z77 mobo with the new r9 series from radeon is running in x1 mode for the pci-e and i wonder if i can change it to the 16x capability or if it is auto. (I am hoping you know because you came up with this last great answer when i was on google search)