If you don't see the option to install Skype for Business in Office 365, there are two possible reasons why:
Your business is using Office 365 ProPlus. With Office 365 ProPlus, you download and install Skype for Business at the same time you download and install Word, Excel, PowerPoint, etc. It's all installed as one package of software!
If your business bought Office 365 ProPlus you will have to upgrade to the entire Office 2016 suite of apps in order to install Skype for Business on your PC.
Or, your Office 365 administrator hasn't assigned a license to you so you don't yet have the option to install Skype for Business. Contact the person who gave you your Office 365 sign-in information and ask them to check that a license has been assigned to you.
Check whether Skype for Business is already installed on your computer
If you already installed Word, Excel, and other Office apps on your PC, here's how to check whether you have Skype for Business, too:
Install Skype for Business with Office 365 ProPlus
If Skype for Business and the other Office apps aren't already installed on your computer, download and install Office using Office 365. This way, you'll install Word, Excel, and Skype for Business all at the same time.