If you need to make changes to OMERO.web itself, then you can perform a developer install of omero-web. You need to be within a virtual environment with omero-py installed as described at /developers/Python. Then:
$ git clone https://github.com/ome/omero-web.git
$ cd omero-web
$ pip install -e .This will allow you to edit and run the source code without a build step.
You can then run OMERO.web as described at /developers/Web/Deployment. You may need to restart the web server after saving changes, particularly for python files, before refreshing the browser.