Dear all those affected users,
I finally found a solution.
I was also one of the affected like you do with this problem but now the problem is SOLVED..
Till today's date (27th July, 2018), Microsoft has released several updates.
Step 1: In Windows Update section, "set to automatically install updates".
Step 2: Click "Check now" to search for the latest windows updates.
Step 3: Click on all those important updates to be downloaded and installed onto the system.
Step 4: Do NOT click or (untick) the problematic update to be installed -- Security and Quality Rollup updates for .NET Framework 3.5 SP1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, and 4.7.2 for Windows 8.1, RT 8.1, and Server 2012 R2 (KB 4340558).
Step 5: After all the important Windows updates are downloaded and installed successfully, restart the system.
Step 6: Click "Check now" again in the Windows update to search for updates.
Step 7: This time ONLY Download and install an optional update which shows on the screen July 18, 2018—KB4338831 (Preview of Monthly Rollup). After downloading and installing, restart the system.
Step 8: Now after successfully installing KB4338831, it should fix the problem. Now you can successfully install Security and Quality Rollup updates for .NET Framework 3.5 SP1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, and 4.7.2 for Windows 8.1, RT 8.1, and Server 2012 R2 (KB 4340558) without any problem.
Quick fix is to install KB4338831 before KB4340558. That's how it works...
Hope I solve all your problems and this thread could be SOLVED !
Javz