There exist a number of ways you can choose to approach this. If you have an archiver such as 7zip or WinRAR, you can unpack the entire contents of the UBCD ISO to your harddisk. Suppose we unpack it to C:\ubcd.
Once unpacked, browse to the following directory...
There should be a file named "dosubcd.igz"....replace this with the newer version provided in the link I provided above (you'll need to unzip the archive).
Once complete, browse to...
There's a batch file named "ubcd2iso.cmd". In order to re-master the ISO, you'll need to open a command prompt and browse to the file location and type
UBCD2ISO c:\test c:\ubcd50b4.iso
Done. Alternatively, you can use a program such as MagicISO to open the ISO and "drag-and-drop" the newer file into the image.