See changelog section of README for details (including a database change made by the upgrade).

As always, don't download the source code, you need the release files, one of:

  • uk.artfulrobot.civicrm.gocardless-1.9.1.tgz

Note: this is identical to v1.9, but corrects a packaging/publishing issue, you should upgrade to 1.9.1 immediately if you have 1.9