Once you fill out your personal details on the registration page (including uploading your resume), this resume will automatically be uploaded to the 'resume' section of each application that you submit.
Because of this, if you are applying for new positions, the old resume that you originally submitted will be visible on any new applications.
If you are wanting to update a resume, then you can either click the 'browse' button on the resume section of your application and then upload a new resume. Or, you can go to the person details page on your profile, scroll down to the 'resume' section and click the 'edit' button. Doing this will bring up the file browser window and allow you to select a file to upload.
Cover letters are job specific. Which means that a cover letter is specific to an individual application and is separate to your profile. This means that once a cover letter has been uploaded and the application submitted, you will be unable to update the submitted file. As a cover letter is specific to each application, this section won't automatically be filled in when you apply for a role, so you will need to upload a new cover letter for each position you apply for.