Add directions on how to upgrade gitlab-shell in the upgrader guide.

doc/update/upgrader.md

@@ -41,6 +41,16 @@ To make sure you didn't miss anything run a more thorough check with:
 
 If all items are green, then congratulations upgrade is complete!
 
+### (optional) 5. Application status check fails
+
+If `gitlab:check` task reports old version of gitlab-shell and recommends upgrading, upgrade gitlab-shell by running:
+
+```
+cd /home/git/gitlab-shell
+sudo -u git -H git fetch
+sudo -u git -H git checkout v1.9.4
+
+```
 
 ### One line upgrade command