If the old Super Admin is no longer with the company and did not change the Super Admin role to someone else prior to leaving, please make sure their email account is still active.
Your IT Department can change the login for the old Super Admin and go through the password reset process for their Foxit account by navigating to account.foxit.com and going through the "forgot password" steps.
Once the Foxit Account password has been reset, the IT department can log into the account in Admin Console and go through the steps below to change the Super Admin role:
https://kb.foxitsoftware.com/hc/en-us/articles/4415134346516-How-to-Change-the-Super-Admin