First, navigate to the “Reporting” section of your desired giveaway. Go to Account > Entries.
You can manually select as many winners as you want (depending on your plan). First select the person/s that you want as a winner. Then click the “Pick Winner/s” button.

There are limits on how many winners you can pick per giveaway depending on your plan.
Free plan: 5 winners
Pro plan: 100 winners
Business plan: 500 winners
Premium plan: 2,500 winners