You don't need to amend for the missing 1099-MISC if it was already included your business income. However, as you stated, it is an option for the missing expenses. It could make a difference in your tax liability, especially the business expenses due to self-employment taxes.
Amending is done on Form 1040X. Also, if your return
is accepted, you should not try to amend your return until it has been fully
processed and you have received your refund or your payment has cleared.
Amended returns have to be printed and filed by mail. It
can take the IRS up to 12 weeks or longer to process them. If you are
due a larger refund than on your original return, your amended return should
only show the difference and you will receive a separate check for it. If
you owe money on your amended return, it will show only the new amount owed,
you will have to mail a check with the return.
How to Amend?
https://ttlc.intuit.com/replies/3288565
