I just received my 1099B and I already filed my taxes and received my return. How do I proceed?

You cannot change or add anything on the return that you just e-filed, nor can you stop it.  It is too late, just like when you put an envelope in a US mailbox on the corner.  The IRS does not allow you to take it back.

If you left out a W-2, a 1099G, or a dependent, or a 1099 etc…DO NOT change your return while it is “pending.”  The changes will go nowhere.

Now you have to wait until the IRS either rejects or accepts your return.  If your return is rejected, you will be able to go into your account and make the necessary changes to your tax return and re-submit your return. 

 If the IRS accepts your return, however, then you have to wait longer until it has been fully processed and you have received your refund.  THEN you can prepare an amended tax return and e-file or mail  it in. You have to be able to work from that return exactly the way it was when it was e-filed originally.  You will need to use a form called a 1040X.

  Meanwhile, DO NOT go in and start changing anything on your return in the system, or you will make a mess for yourself.  Sit tight and wait until you see what the IRS does with the return you just e-filed

The Form 1040X you might need becomes available in mid February,  But wait until your return has been processed.

Form 1099-B includes your capital gains and losses from the sale of stocks, bonds, and other securities. It also can include 1099-INT (Interest Income), 1099-DIV (Dividend Income), and sometimes a 1099-MISC (Miscellaneous Income).

For more information on broker transactions, ‌ refer to the TurboTax article What is Form 1099-B: Proceeds from Broker Transactions?

Since you've already received your refund, then your return has been processed so you can amend your tax return to include your 1099-B.

For more information and instructions, review the TurboTax articles  How to Correct Federal Tax Returns , How to File an Amended Tax Return with the IRS and  Video: How to Amend Your Tax Return.