You may refer below link to download the next version that you want to upgrade.
https://www.virten.net/vmware/nsx/vmware-nsx-t-release-build-numbers/
Login to NSX-T console using Admin account
System -> Under Lifecycle Management -> Click Upgrade
Select MUB file path that you have downloaded ->
Download the supported MUB file
Upload the file VMware-NSX-upgrade-bundle-4.1.0.2.0.21761719-le.mub
Click Upload
Now .MUB file is getting upload, wait for to complete upload
After Uploading extracting the bundle and verifying compatibility
Now Extracting Upgrade Bundle
Now checking Compatibility Matrix of the Upgrade Bundle
Post all validation completion
Click on Prepare for Upgrade
Accept the License Agreement
Preparing for Upgrade
Click Run Pre Checks
Pre-Check is running
Once Pre Check complete
Click Start Upgrade
Now Upgrade has completed
Now wait for complete the Upgrade
Upgrade completed with new version NSX-T 4.1.0.2.0.21761719