Hi guys. I don't know if this has been posted already. After a clean install of Office 2013 this problem disappears and it sets the icons to default. BUT, if you did an uninstall of the preview version of Office 2013 and installed the RTM follow this guide to enable the buttons on the links shown on the title of this post.
You have Mail, Calendar, People, Tasks showing. You want icons instead of the links.
1. Click on the three dots at the end of the links of the first image.
2. Click Navigation Options.
3. Check "Compact Navigation"
4. Click OK.
And you're done!
Hope this helps!