updating contents of a package

I read through the "Writing R Extensions" document and am able to now create
my own packages/libraries which so far are just well documented collections
of my own R functions. I use package.skeleton() and the tools package to
build these packages.
However, it is not clear to me how to modify and update a package after its
initial creation. How do you elegantly update e.g. the old help file when
one added an argument to a function ? How do you keep most of the existing
package structure when implementing incremental changes ?