How to update without new installation?

How do I update to a new version of R on Linux without going

through a new installation?

Most kinds of Linux use some sort of binary package like rpm.
Several of those are available on CRAN, specifially
http://cran.r-project.org/bin/linux

If you can do that, "going through" sounds harsh.  You download
the rpm file and give one command, such as rpm -Uvh r-base...rpm
and you are done.  If you have some version of Linux other than
Debian, Mandrake, RedHat, or Suse, then you need to compile R
from the source, as you would do on Unix.

How do I update packages?

Start R as root and type
update.packages()
See the help about update.packages for the various options.

Jonathan Baron, Professor of Psychology, University of Pennsylvania
R page:               http://finzi.psych.upenn.edu/

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