Ultimately I see two ways forward.
The table comes off, goes on a large milling machine and gets machined straight, parallel, even.
That is probably expensive and hard to organise.
DIY method will need an accurate straight edge clamped to the table, parallel to the blade, as your reference. Then you will need to correct the slot by hand wherever it needs it, using a fine file with a safe edge, a diamond sharpening stone or other abrasives.
You will need to map the error as
@ChaiLatte describes above to know where to remove metal.
It will be a bit slow. Take a bit off, measure, take some more.... until you have the slot as straight and parallel as you want it.
Concentrate on keeping the sides of the slot vertical, not sloping or undercut.
Minimise the amount you remove and then adjust the bar of your mitre gauge to fit smoothly but snugly in the slot. Some bars are adjustable width, albeit to a limited degree.
Your saw may have some way to adjust the mitre slot parallel to the blade. They usually do, even if just some slop in the bolt holes that connect the top and the spindle assembly. If so, you only need to get the mitre slot straight/parallel. You can realign it to the blade afterwards.
Oh, and sharpie marker on the sides of the slot, then file etc. Repeatedly so that you can see that you are removing metal where you intend to.
