We always need more help improving Octave and there are many ways you can contribute. You can help by fixing bugs, developing new features, writing Octave packages, answering questions in Discourse, helping to improve the web pages. If you are wondering what to work on, we have a standard answer:
What would you like to work on?
Using the Development Sources
The latest development sources of GNU Octave are available via Mercurial (hg). The primary repository address is https://www.octave.org/hg/octave, which currently redirects to https://hg.savannah.gnu.org/hgweb/octave. You obtain the latest development version of Octave sources with the following command:
hg clone https://www.octave.org/hg/octave
The Developer FAQ on the Octave Wiki contains more detailed and recent information about Octave development.