How to handle / debug error messages?

Hello,

on help.macbay.de I have enabled debug mode to see if any issues exist and I found two:

1.
Notice: add_object_page is deprecated since version 4.5.0! Use add_menu_page() instead. in /home/staff/websites/hilfe.macbay.de/neu/wp-includes/functions.php on line 3707

2.
Notice: Undefined property: WPInfobox::$include_css in /home/staff/websites/hilfe.macbay.de/neu/wp-content/plugins/wp-infobox/wp-infobox.php on line 252

I wonder how to handle these and if you can help me with it?

Thank you and kind regards,
Sascha