Hello Lionel Mauriéras,
I could not find an update numbered KB31386123 in the Microsoft Update Catalog. However, Microsoft does list KB3138612, which is an update for Windows 7 Service Pack 1. It is possible that KB31386123 contains an extra digit.
You can check and download KB3138612 from the Microsoft Update Catalog here:
https://www.catalog.update.microsoft.com/Search.aspx?q=KB3138612
Before installing it, please confirm that KB3138612 is the update you intended to find. Could you please confirm where you found the KB31386123 number, such as in an error message, update history, or third-party instructions? This would help verify which update you need.
Although Windows 7 has reached the end of support and no longer receives regular security updates or technical support from Microsoft, you can still try downloading and manually installing this archived update from the Microsoft Update Catalog. However, installation and compatibility cannot be guaranteed on an unsupported operating system.