Neutron Single Event Upset (NSEU) how its effect is mitigated in electronics in space vehicles?
There are several aspects.


Neutrons (or high energy radiations) can wear materials by moving atoms or making transmutations... and the electronics may eventually fail permanently.
There can be also transitory perturbation where the effect is merely that some electric charges are displaced making a glitch in circuits.


There are a few mitigation techniques:


Redundancy. Several devices doing the same thing. Sometimes only one is powered to save energy : "cold redundancy". (as a comparison, in aeronautics, there is also redundancy for safety, but all systems are powered : "hot redundancy" for a faster reaction time and because power constraints are less strict)
Specialty technology. For semiconductors, some techniques are less sensitive to aggression : Things as SOI, "sillicon on saffire" are better because the volume of material where a particle can have an effect on the circuit is smaller. There are many other techniques, I'm not an expert. Nowadays there is a trend to limit the use of funky materials to save money : More redundancy, more fault tolerance instead of unobtainium components.
Error Detection and Correction and triplication techniques. By using error correction codes (hamming, Reed-Solomon...) you can be able to detect and correct errors in memory areas, registers.. By triplicating your design and using voters, you can be tolerant to a single upset somewhere.
System design. Be able to automatically reset and restart your computer without compromising the mission. Independence of functions. Self monitoring...


Finally, there are statistics. The radiation flux is different in aeronautics, low earth orbit, deep space probes and probes getting close to the sun. You get failure probabilities, FMEA analysis, and over-design enough to get an acceptable probability of failure.