@dentalgirl39  you should be able to download it from the app store. Please see the link below for full instructions. 

https://answers.webroot.com/Webroot/ukp.aspx?pid=17&app=vw&vw=1&solutionid=1970&t=Install-SecureAnywhere-Mobile-SecureWeb-or-BackupSync-for-iOS#:~:text=Open%20the%20App%20Store%20on,Tap%20to%20install%20the%20app.&text=Enter%20your%20Apple%20ID%20password,icon%20on%20your%20Home%20panel.

As @Martin.1 has mentioned - access the app store and you want to download Webroot Mobile Security

https://apps.apple.com/gb/app/webroot-mobile-security/id460986853

This will then open a “browser” sort of interface which will ask you to login to the account that has your paid license/account subscription.

It should just run through all it’s options afterwards.

So be aware that the Webroot app for the iPhone (iOS) does not do much compared to the Android version. The security model inside iOS does not allow one app to scan the files of other apps, so Webroot Security cannot do that on an  iOS device. Webroot Mobile Security for iOS is basically a very secure web browser.  There is also a second product,  Webroot Wifi Security & VPN which adds a VPN service to your iPhone (or iPad) for use on insecure networks. 

As stated above, simply open the App Store app on your phone, an search for “Webroot”, and you will see the Webroot Mobile Security app. Download it, and launch it. That simple.