The first approach is to look at the downloads page on the developers' website, in this case
http://www.gnumeric.org/download.html - this indicates that you can compile the latest version from source code if you want and are able to.
The next approach is to look to see if there are any other repositories (usually a PPA - Personal Package Archive) with a more up-to-date version of the package for Xenial. In this case, a search for
gnumeric ppa shows that, as far I can see, this is not the case (
https://launchpad.net/ubuntu/+ppas?name_filter=gnumeric).
Which, unless you can find someone else who has produced a more up-to-date version of the package for Xenial, leaves you with the option of upgrading your version of Mint to 19 where the version of Gnumeric is 1.12.35.