In Eclipse/EGit is there a way to edit commit message of unpushed/local commit?

You can select the commit and right click, then:

Modify -> Edit

to rebase onto that commit, amend it and then continue.

That would be the "Amending Commits" section of the EGit User Guide:

open the Staging View or Commit Dialog again and select the option Amend previous commit in the toolbar.

See also this tutorial:

Git amend allows to adjust the last commit.
For example you can change the commit message.
The Git Staging view allows you to perform the Git amend command via the highlighted button in the following screenshot.

See more about git commit --amend in general at "The git commit --amend Command".

99sono adds in the comments:

I use:

Team Synchronizing Prespective > History View > Modify
(submenu) > Reword .
Finally this opens a popup where I can edit the old commit message.

See "Add ability to reword and squash commits directly from the history view"