Add instructions on how to update existing conda environment

[tobiasdiez]

📚 Description

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

📝 Checklist

• I have made sure that the title is self-explanatory and the description concisely explains the PR.
• I have linked an issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: 23da0c6

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (c00e6c2) 88.62% compared to head (23da0c6) 88.61%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #35352      +/-   ##
===========================================
- Coverage    88.62%   88.61%   -0.01%     
===========================================
  Files         2148     2148              
  Lines       398855   398855              
===========================================
- Hits        353480   353450      -30     
- Misses       45375    45405      +30     

see 29 files with indirect coverage changes

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