gh-35352: Add instructions on how to update existing conda environment
    
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### 📚 Description

Add instructions in the docs on how to update existing conda
environment.

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URL: #35352
Reported by: Tobias Diez
Reviewer(s): Matthias Köppe

Author: Release Manager

src/doc/en/installation/conda.rst

@@ -173,3 +173,7 @@ suffices to restart Sage.
 After editing any Cython files, rebuild the Sage library using::
 
   $ pip install --no-build-isolation -v -v --editable src
+
+In order to update the conda environment later, you can run::
+
+  $ mamba env update --file src/environment-dev.yml --name sage-dev