My Windows 10 system showed 1.1.13701.0 but Windows Update indicated everything was up to date. I clicked on the usual Windows Update button Check for Updates anyway and now Defender shows 1.1.13704.0
It'll update on definition update, too, it seems (without going through Windows Update).
They do? Just check the version: Open Defender, go to Help => About, check "Engine Version". Should be 1.1.13704.0 or higher.
I also find the version under Settings -> Updates & Security -> Windows Defender.
Thant screen gives you the version numbers in the place you're most likely to want to update.
If you assume the instructions for Windows 8 apply to windows 10, then that is what you find. Also, the security advisory itself links to this page with the statement "For more information on how to verify the engine version number that your software is currently using, see the section, "Verifying Update Installation", in Microsoft Knowledge Base Article 2510781.", but the actual section title is "Verification of the update installation", so when I searched for the stated section name, I did not find it. These are small mistakes, but it is sloppy work. A person might be left wondering if there is some Windows-10 -specific information somewhere in the rabbit-warren of links leading from the security advisory.
on my win10 machine, I had to open Windows Defender Security Center, click the cog in the lower left, and then click 'About' in the upper right.