The firmware update is dated May 15, 2018, and the Microsoft support website points out these are the changes:

Surface System Aggregator — 135.2139.257.0 improves docking compatibility.

This is the second firmware update the Surface Laptop is getting in May. Recently, Microsoft released another firmware update for its laptop to improve hibernation also for users running the April 2018 Update. The update should download and install automatically, but if you have not received it on your Surface, then you can go to Settings > Update & Security > Windows Update and try to force the download manually. Alternatively, you can also manually download this firmware from this Microsoft website. All content on this site is provided with no warranties, express or implied. Use any information at your own risk. Always backup of your device and files before making any changes. Privacy policy info.