ruby upgrade rubies (i did ruby-1.9.3-p374 =＞ruby-1.9.3-p545)

rbenv

$ rbenv install --list
$ rbenv install 1.9.3-p545
$ rbenv rehash
$ rbenv global 1.9.3-p545

rvm

# NOTE: you probably want to upgrade your rvm first, as the list of known rubies seems to be coupled to the rvm version.
$ rvm get stable
$ rvm list known
$ rvm upgrade 1.9.3-p545
$ rvm rubies

=* ruby-1.9.3-p374 [ x86_64 ]
ruby-1.9.3-p545 [ x86_64 ]

# => - current
# =* - current && default
# * - default

$ rvm --default ruby-1.9.3-p545

#Note:manual gem installation could be better coz some of them are incompatible with new ruby.
$ rvm all do gem update