if you open the debug toolbar and go and open your debug toolbar using alt + f12 and you go to the gameplay tab from there you can enable wether or wether not your kerbals to respawn or not too, and if you would like to bring them back to life immediately you can go to your "saves" folder in your ksp directory go to persistent.sfs file in one of your saves then you just turn the dead kerbal's ... More @Wikipedia
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