MacOS 10.14 ("Mojave") is no longer supported as of January 1st, 2022. You can check your operating system (OS) version on a Mac by clicking the Apple icon in the upper-left of the screen and then clicking "About this Mac"
Step 1 - Visit https://support.apple.com/en-us/HT211683
Step 2 - Scroll to the section “Use Safari to download macOS on your Mac”:
Step 3 - Click on the link for “macOS Big Sur”
Step 4 - The link will ask you to open in the App Store, and will take you to this page within the App Store:
Step 5 - Click “Get” and follow the steps to update your operating system.