When you want to add contact information you have two options:
First of all, you can add these information, while you create a company. There is the pop-up where you got the tab "contact". Further information for that you can find here: How can I create a company?
When you forgot to add them there or just want to add some new contact details afterwards, you have another option. For that you have to go to the company overview and click on the three little dots, which are on the right in the row of the company. Then you can decide between delete and edit. When you click here on edit, there is a new pop-up.
In these pop-up you can edit the main- and contac information. When you switch to the tab "contact" you see the existing ones and you can add some new contact details.
After all you just have to save this information and that's it.