To accept a life you have received, complete the following steps:

1. Go to the restaurant's main screen.
2. In the top right corner, tap on the envelope icon.
3. Go to the Gifts tab, where all the lives you have been sent are stored.

If you have the maximum number of lives, you won't be able to use extra lives. Don't worry, they won't disappear, and you will be able to use them later.