First, to determine if you can take your parent as an exemption, use this tool from the IRS:
If you can claim your parent, then follow these instructions to amend your 2016 return:
If you have already filed your return, first print out a copy of your original return for your records.
Then go into Turbo Tax and click Amend (change) a filed and accepted return under Manage Your Return.
It's zeroed out (which is why you need to keep a copy of your original return). You can then change the number of dependents to include your parent.
You need to wait till your return is accepted by the IRS before you amend.
You mail the 1040X along with any schedules that changed as a result of the amendment (except the 1040).