If by Questions tab you mean the nav bar content item "Questions", you may rename it by changing the lang file content.
To do so:
1/ First copy the "qa-lang-main.php" file located in the "qa-include" folder to the "qa-lang/en-GB/" folder. This way, you will avoid editing the core files, which is not recommended if you have other option. This will override the core content without actually editing core files.
2/ In the "qa-lang/en-GB/qa-lang-main.php" find arround line 118 the lin 'nav_qs' => 'Questions' and replace "Questions" with whatever content you want to display.
3/ Go to admin tab>general, and choose english UK as the language (to activate the language folder you've copied and modified you file in ).
This is assuming default language install and settings, if you have installed custom language files, you may edit the file mentionned above directly in you language folder.
