If you agreed to change your fee to another price suggested by the client, you CAN change it. You can't change anything you have written, but you can change the amount. Is this what you meant by changing your offer?
ETA: If this is what you meant, go to your proposals and you should see this just above your cover letter: http://prntscr.com/ma207k
If you click on "Change Terms", you will then see this - or one that relates to a fixed-price job:
You can then change the amount you originally put.