You must go bankrupt to restart a level.

First you must go through the upgrade section and sell all upgrades purchased. Then go sell all the firms.

Now pay off a loan you don't have the money to cover.

Example: You have $500, your loan amount is $1,500. Go to loan screen and raise daily amount to repay to $1,500, click Accept and you will get message that you have gone bankrupt, then you will have option to replay level.