That does sound correct. Try aptitude. (i.e. "sudo aptitude")

Search for nginx by hitting forward slash for the search box and typing nginx in the box and then press enter when your cursor is on the nginx line. That brings up a full page about the nginx package and lists the available versions at the bottom.

If it doesn't list the 7.61 version then you don't have the repository added correctly to the /etc/apt/sources.list file or didn't do "sudo aptitude update". ("aptitude" can be substituted for "apt-get" for most command line operations and can be more convenient) Even if you failed to add the authentication key correctly it should still work, it would just throw various warnings about untrusted sources.

If it lists both versions then you can select the 7.61 version and + it.