Yes, you need to amend your return to show that you are not claimed as dependents on someone else's tax return.
How to Amend Your Return Using TurboTax Online
Once you have made the change, you will need to print, sign, date and mail your amended returns according to the instructions that print out with the returns.
Amended returns cannot be efiled.
What do I need to know about the coronavirus stimulus package?
How it may affect your stimulus check is difficult to say since some of the parameters are not fully known.