Provided that the tender event is still open (within the established timeline) you are able to modify your quote even after submission.
For the purpose you just need to open the RFQ your would like to modify.
In the bottom right corner you will see the Revoke button.
Once clicked your quote submission will be revoked, and the submission button will reappear.
Now you are able to modify your initial submission. Note that any valid lanes that are already on the rate sheet will remain in the cache of your quote, thus if you add additional lanes they will be added on the top of the already validated ones.
To erase the already validated lanes and have a brand new upload you would need to click the reset button and the proceed to upload your rate sheet.