Think about how the users in your domain would say it, and what the domain default is. They'll be coming to your site with the model/familiarity of similar sites they've experienced.
Generally speaking, in content based (articles / news) websites, you'll see the term 'update'. In the content / news domain it implies adding to understanding based on new information.
Modified often refers to an entity or record, and is more associated with technology, the act of manipulating objects. It does not necessarily always imply new, useful information, but a change in an objects properties or values.
Most of all, think of how your users would speak.
A thought on time
Which ever term you go with, you can use relative time, so users don't have to do as much mental math.
Depending on the frequency / importance of new information being added to the article, you choose the granularity that is easiest to understand.
- 30 minutes ago
- 6 hours ago
You also might drop the the 'Last', and just say:
Updated: yesterday or Updated 6 hours ago