You would change it on Form T2125. There are different ways of accessing it depending on which version of TurboTax you are using.
If you're using TurboTax Online:
Click on “Self-Employment” under the Income tab.
You can also click on the “Find” icon in the blue menu and search for “Self”. Select “Self-Employment” and then click the “Go” button.
If you're using TurboTax CD/Download:
If you are using the EasyStep Interview method, click on the “Find” icon in the blue menu. Search for “Business”. Select “Business Profile” and then click the “Go” button.
If you are using the Forms method, click on the “Forms” icon in the blue menu. Search for “T2125”. Select “T2125 P1” (or which ever has the business you want to edit) and then click the “OK” button.
If you need to look up your industry code, you can do on the CRA website: http://www23.statcan.gc.ca/imdb/p3VD.pl?Function=getVDPage1&db=imdb&dis=2&adm=8&TVD=118464
Or you can use the search tools on the NAICS website: https://www.naics.com/search/