You can modify the reassembly timeout by making suitable change in these lines i.e.,

if(frag_info[i].len > 0 && timer_expired(&frag_info[i].reass_timer)) { clear_fragments(i); }

timer_set(&frag_info[found].reass_timer, SICSLOWPAN_REASS_MAXAGE * CLOCK_SECOND / 16);

You can find these lines in sicslowpan.c of contiki 3.1 version

https://github.com/contiki-os/contiki/blob/release-3-1/core/net/ipv6/sicslowpan.c

For more clarification you can explore: add_fragment() method or search reass_timer in the file.