There are three ways to set a company's vertical.
Table view (drop down) :
Go to Contacts > Companies, then click the drop down for the company you want to Edit and click Edit
Click on the drop down menu for vertical. Select "Manage Verticals" to add new Verticals if needed
Click Save
Table view (column edit) :
Use the Verticals column in the table view to update a company's vertical. You can also bulk-assign verticals to edit or add verticals to groups of companies.
Company Page:
Go to the Companies page and click on a company's name
Select Edit in the upper right hand corner
Click on the drop down menu for vertical. Select "Manage Verticals" to add new Verticals if needed
Click Save