Get the code. The best way to do this now for Unix users is via Git following the instructions at: http://genome.ucsc.edu/admin/git.html
Or, fetch the entire source in a single file:
rsync -azvP rsync://hgdownload.soe.ucsc.edu/genome/admin/jksrc.zip ./
or: wget --timestamping http://hgdownload.soe.ucsc.edu/admin/jksrc.zip