Unless you have regularly perform a backup of your K2 database, as well as backup a copy of the K2HostServer.exe.config, I don't think there is a way for you to restore the previous setting.
What is the exact issue you are facing now? What kind of permission are you referring to?
It has created a new app pool and using it for management, runtime and designer. The new app pool doesn't have all the permissions and giving weired errors.
Does your original app pool still exist in your IIS Manager? You can always reassign the applications under your K2 site to use the app pool you want. Certain applications must run under certain app pool configuration, but if you have another environment to reference to, it should not be difficult to manually reassign the app pool.
Thanks, I have done that. I am facing OData issue, when I browse my OData API, i am getting below error
404 - File or directory not found. The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.