Of course you can't update from the GUI as this is a pre-release build as the topic title and instructions at the start of the topic states.
You can only get it by downloading it using the links at the start of this topic.
Sorry but your response quoted above is a bit confusing to me.
I used the GUI to update from 5.0677 to 5.1.864 and it worked fine.
So it works to update to from a released version of AIS; i.e 5.0677 to a Beta, i.e. 5.1.864 using the GUI but not to update the 5.1.864 Beta to a 5.1.874 Pre-release using a GUI?
This is confusing to me because Firefox does not work that way.
For example you can't update from 3.x to 4.xBeta using the Firefox update feature.
However, once you have downloaded and installed the first Beta of Firefox 4.X all all subsequent Betas can be updated through Firefox without having to download the complete executable of new Beta release. I imagine that you will be able to update with Firefox all the way through the 4.xRC and onto the public release of 4.X.
, though only time will tell.