Watir xpath problem!

hi,

i have installed Ruby 1.8.2 version because i need ModalDialog
support.Only this version support modal dialog…and i’ve installed
watir 1.5.2 for same reason…so when i try to access element by xpath
using "ie.element_by_xpath(‘button[@id,‘someid’]’).click, i have
error
message like this.

—> RuntimeError: Requires REXML version of at least 3.1.4. Actual:
3.1.2.1

Please tell me how can i upgrade REXML version to 3.1.4. i have read
some discussion about it but i could not find anything…

if you know answer,tell me step by step plz…

good work