In the situation where you've already submitted your proposal and the closing date is extended, you are NOT required to re-submit your bid.
NOTE: It is still recommended that you find out why the project was extended and if there are any changes or addenda to be aware of that may affect your proposal. Often times a Project Owner will post a Public Notice with additional details, where you can check how to view this from our viewing a Public Notice page.
If you are looking to un-submit and modify your submission, you can do so up until the new close date of the opportunity. (See Can I un-submit my bid once it has been submitted and finalized?)