Try to grab the end of the frayed cable and downshift (if you can) then try to extract the cable head with tweezers, picks, anything you can get into it. You really can't disassemble it to get to it (serious design flaw IMO)

I've had 3-4 shimano shifters come in that have broken or break cables inside of them this past month. Check your cable heads regularly (shift up to largest cog, then w/o pedaling, downshift to slack the cable, pop out the cable to inspect)