To update an existing installation of Gurobi, you just need to follow the installation instructions of the new version. It is not necessary to uninstall the previous version. You should, however, clean up the environment variables that were set during the installation to remove paths to old versions and remove the previous installation directories. This helps avoid possible version clashes or incompatibilities with other tools, IDEs, or configurations.
Updating the license file
Keep in mind that updating to a new major version, e.g. going from 8 to 9, does require a new license file.
For automatically updated licenses (e.g. commercial licenses with active maintenance) you just need to rerun grbgetkey using the existing license code that you can find on your License Detail page. Note that you need to use the grbgetkey executable from the new version for this.
For single-user academic license files, you should create a new license on the personal account page. For other license types, please create a support ticket.
Updating a Python/conda installation
To update your Gurobi Python installation, simply run:
conda update gurobi
If you installed gurobipy in your own Python installation, you need to repeat the steps in the Quickstart Guide for the new Gurobi version (python setup.py install for Linux and Mac or pysetup on Windows).