Given an example WU log, follow these steps to diagnose the issue:

1. Filter the log on “FAILED” (case-sensitive) and pipe/redirect those lines to a separate file. This could be done in Powershell as in this example:

Select-String -Path "*.log" -Pattern "FAILED" -CaseSensitive > failures.txt

One other tool that can also be used for viewing Windows Update logs and filtering them is CMTrace, which is part of the Microsoft System Center 2012 R2 Configuration Manager:
https://www.microsoft.com/en-us/download/details.aspx?id=50012

Under the Tool menu, you will find a filtering option.

2) Results look something like this:

3) First focus on the most frequently occurring error codes between the brackets then the least frequent. Also notice that all the error codes are in the same column so it’s easy to identify which ones occur more frequently.

In this case, the 2 most frequently occurring errors are 8024000C and 80246013.

4) Perform a Google search on the error codes and prioritize solution with Microsoft website recommendations first, then preview others high on the hit list. You can also reference Microsoft’s Error Code list here for a general description of the error:
https://support.microsoft.com/en-us/help/938205/windows-update-error-code-list

In this case by performing a simple Google search, we find that these are the most likely issues based on the search results:

8024000C: WUA corruption or issues with the system

https://answers.microsoft.com/en-us/windows/forum/windows8_1-update/getting-error-code-8024000c-when-trying-to-install/3ca8e3d3-57df-46a6-ae8d-8e34ba7a4c72

80246013: Issue with Background Intelligent Transfer Service and corrupt data as well as permissions issue, which doesn’t allow the update to install itself.

https://answers.microsoft.com/en-us/windows/forum/windows8_1-update/getting-an-error-code-80246013-failed-to-complete/0d18f6b3-4878-4631-80e1-aed9cb1ec717

5) It is also often recommended to run the Windows Update Troubleshooter:
https://support.microsoft.com/en-us/help/4027322/windows-update-troubleshooter

This tool can often diagnose the issue and sometimes even resolve the issue(s).

Here’s an example of the Windows Update Troubleshooter finding and fixing 3 issues on a Windows 8.1 system:

Other Helpful References

The interface looks like this when you run it (choose option 2 to reset WU components):

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