I don't see any install new plugins link or any of the installed plugins.
Check that you've not deleted the install folder and that it has proper permission. https://www.socialengine.com/support/article/5112151/se-php-optimized-file-and-folder-permissions
Oh and that mod_sec is not enabled on the server. SE doesn't work with mod_security. That can tend to break things in the ACP and your host would need to tweak it if you want to use mod_sec.
I got it. Now I can do that but where is the core file I supposed to use for the upgrade. Are the instructions talking about just the latest version tar file?
It's the latest version tar file as shown in the screenshot for upgrading. Though it would show an older one, it's the same sort of name.