Changing the gems version for ruby1.9 (in Ubuntu)

I need a newer version of gems visible to my ruby1.9 installation.
Ruby1.9 came bundled with an old version of gems.

I have another gem installation which is visible to my ruby1.8
installation that would work, but I can not figure out how to do either
one of the following:
a)set the 1.9 ruby to use the other version of ruby gems that I have
installed on my computer
b)upgrade the gems currently working with ruby1.9

when I try to run:
$sudo gem1.9 update --system

I get the following:
ERROR: While executing gem … (RuntimeError)
gem update --system is disabled on Debian. RubyGems can be updated
using the official Debian repositories by aptitude or apt-get.

but this works fine:
($sudo gem update --system)