How to replace corrupt files with the backed up ones?

[Lilttlekerbal99]

Hello there I and I have a question,

How do I replace my corrupted file with the back up ones? 

And what does this mean? "Error Accessing to this save, Object reference is not set to an instance of an object'

[diomedea]

As KSP doesn't allow to directly manage backup files ingame, one has to move (better: copy) the one desired from the saves/<game>/backup folder outwards to the saves/<game> folder. Then, to make KSP load it when asked to start an existing game, its name has to be changed in "persistent.sfs", therefore overwriting the persistent.sfs with the error.

About the reported error: the savegame seems be missing one of the very many objects KSP deals with, while loading them and not finding one as expected an error is shown. Can't tell what is missing unless that savegame (and best, also the output_log.txt/player.log) are uploaded to some file hosting service and then linked here for anybody to check.