Opera 42.0.2393.351 fixes a handful of bugs, several related to the 64-bit version of the browser. See the change log for details.
Opera 42.0.2393.517 fixes four more issues, some of which first appeared in 42.0.2393.351. See the change log for details.
None of the changes in either new version seem to be related to security. Opera will usually update itself shortly after a new version becomes available.