How do I upgrade to the new version of TBufDataset with Lazarus 1.2.6?
You can download the newest Lazarus 1.6.4 from the
Lazarus homepage.
I'm not sure if you can just install over your current 1.2.6 but you can uninstall that one and install the new 1.6.4.
Also notice that the default download for 1.6.4 now matches your OS bitness. So if you have a 64 bit Windows you now default get a 64 bit Lazarus. If you still want the 32 bit version (for compatibility issues or otherwise) you can click on "Other" and choose your version.