If you are signing in on a PC that isn't yours, press Ctrl+Shift+N to open an incognito window to prevent caching your login information. And don't forget to close the browser when you're done performing the steps below.
If you are unable to get into the security info page, since you don't have access to the old device that had the original authenticator app, email email@example.com so that we can adjust your account in order for you to proceed with the steps below.
Download Microsoft Authenticator to your phone
From your device's app store, install the app: Microsoft Authenticator. The first time you open the app, allow the use of the Camera and choose the option to scan a barcode. Proceed below to get to said barcode and allow Notifications if prompted.
You should already have the Microsoft Authenticator app installed. If not, we can re-push it to you and you will be prompted to sign in with your Apple ID.
Setting up the app
Copy and paste the following link into your Edge, or Chrome, browser (not Internet Explorer):
If you click the link, it may open in Internet Explorer, hence copying and pasting into Edge/Chrome.
Note: When setting up your Authenticator app, on your phone you will be prompted to allow the camera, scan a QR code, and allow notifications.
First, delete any existing Authenticator app(s) listed, and then click + Add method
From this point forward, keep clicking the blue buttons, and follow the steps it tells you to perform.
On your phone, open the Microsoft Authenticator app, tap "I agree"
Choose the option to scan a QR code (allow the camera). If you don't see the QR option, choose Work or school account.
After you scan the QR code, proceed with the blue buttons on your PC screen.
Last but not least, check that your default sign-in method app shows "Microsoft Authenticator - notification" as shown highlighted below:
That's all there is to it. You can now remove the old LastPass Authenticator app since it has just been replaced.