Before allowing anything and overriding a security system - you need to satisfy yourself that it has not been compromised. You could do some Google searches on the domain, perhaps check with the site operator if they know why Umbrella would be listing it as having been used for phishing. Ideally, have the site owners run an external security scan.
Umbrella does not usually get phishing warnings wrong. You should err on the side of caution.
Next, if you are satisfied Umbrella really is wrong, you can't override anything with a free Umbrella account. You need a paid account to be able to whitelist domains.
From reading further down, it sounds like you are using a free account. That being the case, I only see two options:
1. Stop using Umbrella temporarily.
2. Change to a paid account. You can get more info at https://umbrella.cisco.com/
Also, I would take extra special precautions accessing this site for the moment (assuming you override Umbrella) because you are increasing your risk posture. Let users know there is an elevated risk accessing the site and they should be extra careful clicking on links to verify if they are real or fraudulent, and to be careful of anything asking for passwords, personal information, etc. They should be looking for anything unusual for differences in behaviour.
I would also double-check the anti-malware on the machines accessing the site are right up to date.