diff --git a/textbook/chap6.md b/textbook/chap6.md
index 6605329..dd5e796 100644
--- a/textbook/chap6.md
+++ b/textbook/chap6.md
@@ -38,4 +38,11 @@ or
 
 ```
 python manage.py test lists
-```
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+```
+
+## 6.2 Aside: Upgrading Selenium and Geckodriver
+
+Sometimes, mismatch between firefox and (Selenium + Geckodriver) may result failure in test, as firefox may auto upgrade itself at night. Upgrade Selenium + Geckodriver as follow:
+
+1. `pip install --upgrade selenium`
+2. Download new geckodriver
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