How to save something that I do on nifi.?

The Apache Nifi Registry is another option. You can version processor groups, commit changes to the registry, revert back, add comments, etc. I see it as similar to version control such as Git. It's helpful in collaborative settings as well as promoting changes from development to production.

Apache NiFi automatically saves your flow for you. The flow configuration file is stored in ./conf/flow.xml.gz by default. NiFi also maintains an archive of previous versions in ./conf/archive. See the Core Properties settings for changing these settings. It is possible to backup and restore the flow configuration through these files.

Apache NiFi also has a Templates feature that makes it easy to save a selection from your flow, export it from the UI, and share it with other NiFi installations. Templates may be a closer approximation of a "save" button.