Keep Secret does not have a «Log out» feature. To sign out of your Keep Secret account, you need to uninstall the app itself.
It is NOT recommended to delete Keep Secret: all files that are not synchronized with Cloud Storage will be permanently lost.
If you still decide, then make sure that all files are synchronized with the Cloud and only then start to uninstall the application. When you log back into your account, only those files that were backed up in Cloud Storage will be restored.