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Have a look at the template section in the BE Menu. Look if you find there an Object named TMENU. Is so, this would be your solution:
ATagParams = title="{field:abstract // field:subtitle // field:title}"
ATagParams.insertData = 1
Like that first the abstract is taken, if empty the subtitle, if empty the title.
Have a look here: http://www.serious-cool.de/webdesign/typo3/dynamische-linktitel-im-menue/
I you cannot find anything alike, your template is propably rendered with fluid. That means you have somewhere in your system a navigation.html file you need to find and modify.
Since Typo3 is very dynamically developing there are more then one possibility to render a menu....