Shimano FC-M521 middle chainring replacement
According to the Big Blue Book of Bicycle Repair, Shimano steel chainring bolts 70-95 inlbs. Aluminum 44-88 inlbs.  A Park Tool CNW-2 chainring tool or similar is used to hold the slotted nut and an allen wrench turns the bolt. Depending on the size of the chainrings you may be able to use a slotted screwdriver to hold the nuts.