You can update job details by pressing the "Update" button on the Jobs tab of the related project or right on the job page:
It will open the Update job page where you can make the necessary amendments:
Press "Update" to confirm changes.
Note: The Language pair and Vendor fields cannot be changed if the job is not in the Draft status, and\or it has payables that are linked to the current vendor.
So in this case, to be able to change the assigned vendor and language pair, you should delete payables and\or change the job status.