As a user, once you submit a form, you cannot edit the fields in that form any more. 

However, here are some other options you can explore:

This option works best if you were the one who started the item and it hasn't gone through any other workflow tasks yet. 

On the left side of your dashboard, in the My Items section, click on In Progress. This will show you a list of all the items you started. Click on the item you want to change. At the bottom, there is an option to Withdraw the item. Confirm that you want to withdraw it. 

An email notification will be sent to you and to anyone who has acted on this item so far, including the current approver. 

After withdrawing the item, you can start a fresh request. You can only withdraw items you initiated. If someone else started the item, and you want to edit your responses, you'll have to use another option.  

However, the Super Admin has the ability to disable withdrawing for users. If you don't see the option to withdraw, contact your account administrator. 

Whoever is an administer of this app is able to edit the form at any time. Follow this guide if you are App Admin. 

You can find out who the next person in the workflow is by checking out the progress bar for the item. Send a note to that person and ask him/her to reject the item back to your task. You will get an email notification once the rejection happens.
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Only Approval tasks have the option to reject. If the next step is an input task, the task owner won't be able to reject the item back to you. 

If the item is in the middle of a workflow, it will show up in My Tasks at the same place where it was before. You should be able to work on the form as usual. 

If the task is rejected back to the initial task, it will show up on your My Items under Rejected. You can make changes to your form and click Restart. Restarting an item can save you time since you don't need to re-enter all of the form data. Simply edit the fields you want to change and submit the form again.