When Jack steals Manfred's glasses and puts them on, he somehow "becomes" Manfred from an "information dump" that comes.
But that doesn't make any sense because unless I'm mistaken, the glasses just display image and text no?
For most people yes they do only display images and text but Manfred has become so heavily dependent on his data feeds that his specs run his life. As such when Jack puts them on and starts to follow their advise he becomes the man whose life they have been programmed to run. It's not that his specs do anything different to anyone else's its that they do more, much more, of the same, to the point of almost being the person they work for.
I should probably start by linking the wikipedia articles for Virtual_assistant and Augmented_reality, as the glasses in Accelerando use a combination of the 2 technologies. Development in this area is ongoing today, and products similar to Manfred's glasses are now beginning to be realized in real life commercial products today. It is important to note that in the book, Manfred is a software developer, and has personally developed his own system, that is more comprehensive than what is generally available to the public, not to mention personally tailored to Manfred.
During the writing of Accelerando, these technolgies were still in their infancy. When the book was published in 2005, Google Glass did not yet exist, nor did Oculus Rift, or the iPhone for that matter. So when writing the character of Manfred Macx, Charles Stross had to use his imagination. In addition he was probably drawing inspiration from researchers like Steven Mann, inventor of EyeTap.
Ironically, events from the book somewhat echoed themselves in the real world in 2012 when Mann was assaulted while eating in a McDonalds in Paris with his family. The assailants were thwarted though by the fact that Mann's EyeTap was surgically bolted to his skull, and the would be muggers were unable to remove the device from his head.
Unfortunately in the book, Manfred did not have this foresight, and his glasses are swiped, leaving him temporarily incapacitated. It seams plausible that Manfred might be so incapacitated given his excessive reliance on them in the book. In the real world there is at least one report of someone becoming addicted to Google Glass in 2014, and subsequently going through withdrawal. Also internet addiction has been well documented for some time now.
As for Jack's ability to use the glasses. In the book, Manfred doesn't have a password or any biometrics linked to his glasses. This is most certainly for comic effect, as most modern devices these days already have these security features. But without this security hole, there wouldn't be much of a story if Jack had been locked out of the device. And there wouldn't have been the opportunity for comic relief when Manfred's Cartesian theatre AI has trouble interpreting the unfamiliar voice data from Jack(yes, if you read, you will find that he speaks to the glasses, and they speak back). Jack goes to Manfred's meeting in his place, in a misguided attempt to extort money out of Manfred's business contact, but the meeting does not go as expected, in part due to Jacks general ignorance, and inability to pass along information provided to him through the glasses. TLDR: It's not that Jack became Manfred, but more simply that Jack saw an opportunity to make a quick buck, and so followed instructions, only to later find out he was not up to the task of pretending to be someone he was not.