The 2011 version cannot be updated to be 64 bit compliant unfortunately. The difference is just the libraries and dependencies used by each. In an effort to streamline macOS, Apple is no longer supporting the very very old 32 bit libraries and dependencies for these old applications. Transition has been under way for many years now.
You can choose to get the monthly or yearly Office 365 subscription, or the one time purchase of Office 2019 if you prefer.
See here: https://products.office.com/en/buy/compare-microsoft-office-products-for-mac
Note that this is only necessary if you plan on updating to macOS 10.15 Catalina in the fall when it is released. It you plan on staying with High Sierra, then your Office 2011 will continue to work as it has.