First, navigate to the contacts menu. Once in here, find the contact you would like to edit. You can filter the list by either searching by email, name or tag.
Once you found found the contact, click on it.
You can then add the information you want to include in the contact fields.
You can also further add tags, such as their industry, that will allow you to send them targeted campaigns
Once you have finished editing, Click Save and return to Contacts.
Further Reading: