I don't think you can actually do this. I took a quick look at the Breadcrumbs webpart and it inherits the CMSBreadcrumbs object which isn't editable but this might be it: CMSAdminControls/UI/PageElements. Note: If you edit this file and then it gets replaced in an update/hotfix, you'll loose your work.

Here are some quick steps of how to edit a Web Part in the future, though:

First, you're going to want to clone the WebPart. What this does, is create a copy of the WebPart, this way, when you apply a Hotfix, you won't accidentally overwrite your work.

Do do this, go to SiteManager > Development and select WebParts. Scroll down the list, expand Navigation, and select Breadcrumbs. Click "Clone Web Part" above that. In the popup window, give it a new name, new codename, and new filename. Click "Ok."

Now, in Visual Studio, open your Web Solution. You'll find the cloned control in the CMSWebParts/Navigation/ folder. Select the filename you created above, and open the control.

From there, you can edit the code to do what you want it to do.

Finally, use your new Breadcrumbs control instead of the standard one.

I hope this helps!

-Eric