right-click on the "clock" in the panel
Go to preferences (or so... I have a localized version)
There you can adjust the format of the time displayed. ( I like the 24 hour since we here actually use it all the time - as opposed to you english-speaking countries who use 12 hour format and AM/PM to tell what time it actually is
)
[edit]
as for the BIOS: you will need to "think" in the 24 hour format there
The (kind of) sad news is: you probably did not have to replace the battery - but you are good to go for a few more years now...
Always back up - or at least take note of what you are changing - before you do something like that what you did...
