Place a trivet in a multi-functional pressure cooker (such as Instant Pot) and add water. Place crab legs on top of the trivet; you may need to let them thaw for a few minutes so they will fit.
Close and lock the lid. Select high pressure according to manufacturer's instructions; set the timer for 3 minutes. Allow 10 minutes for pressure to build.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid.
Use tongs to transfer crab legs to a serving dish.
Squeeze lemon juice on top and serve with melted butter.