All major and minor versions of NetWitness (11.7.0, 12.0.0, 12.1.0, etc.) can be downloaded either via the built in repo within NetWitness itself, or you are required to go to MyRSA (my.rsa.com), log in to your account, and then download the zip or ISO from there. All service pack and patch releases are available here on the Community.
If you do not have access to MyRSA, you will need to put in a Support case requesting MyRSA access. In that request you will need to provide your email address, your company's name, and a license serial number from your NetWitness environment.