How do I update my galaxy s9 to one ui 4.0
Unless you have the Exynos variant, you can't update your device beyond One UI v2.5
Galaxy S9 end of term ... no more updates
Can I update to one ui 4.1
Why would you think that you could update to 4.1 if you can't update to 4.0, AND the last version of One UI for your device is 2.5?
If you have an Exynos variant, you could unlock the Bootloader & Flash a Custom Rom, but it wouldn't be One UI, it would be an Android version update, and IF a developer has compiled one for your device.