While the Offer Submission Phase is still ongoing, you edit your active offers in a tender. Specifically, you can change the quantity, the price and the validity date. If, instead, you wish to withdraw your offer altogether, please check this article: How to Withdraw a Sale Offer from a Tender?
To edit your active offer, do as follows:
1. Log into your account;
2. Click on "AggregateEU" on the top blue menu;
3. Then, click on "Tendering and Matching" right below;
4. Click on "All Tenders;"
5. Finally, click on "Edit Offer" inside the tile of the relevant tender.
You will then be taken to the offer editing page on which you can alter the following values:
Once you've edited the relevant values, just click on "Submit."