How do is fix my Numark m6 bleeding cross fader

You might call mackie for a suggestion at 800-258-6883. Sometimes there may be older parts in stock at other Mackie distributers around the world. Here's a list of contact info Mackie International Distributors It doesn't hurt to ask and email is free!
I had good luck getting parts for various musical electronics out of Canada

By "fader" we'll assume you mean the sliders on a mixing console, etc. These potentiometers are rectangular in shape and are usually held together by bendable "tabs" that hold the mechanical mechanism together. If you have pushed the control apart, you will need to try and realign the casing and bend the tabs back into place, but be warned. They may also be destroyed and need replacing.

If you aren't using one of the group outs you could re-route the main L/R through one of them. I can't remember if that board will let you configure the MONITOR outs for either PRE/POST fader but if so you could go that route too.

Otherwise that fader is most likely dead & you'll probably have to replace the MAIN L/R, master output module if you can't find a competent tech to fix just the fader.

Changing faders USUALLY involves removing all panel hardware so that circuit board can be removed to get at the fader. It is often a LOT of work with a boit of hazards along the way.
If the control is noisy I want you to try something before replacing the fader. Get a can of CRC226 available at Home Depot in the Electrical department... about $2.70 for a can. Using the included plastic spout tube, squirt a bit into the center of the control where the knob is. Work it back and forth and then put a little more in and test if it is better or fixed.
NEVER use DeOxit or Blue Shower in faders!!!!! It removes the lube that is required in the fader.

Usually a "leak" from one side to the other is an indication of a cracked resistance element. The fader would have to be replaced.

I lube the faders on my mixers using CRC226 availablle in the electrical dept at Home Depot. Use sparingly.

I suspect one end of the crossfader pot has broken from the element. The fader probably costs less than $5.

You likely will have to remove all the knobs and some screws to seperate the board with the fader from the front panel so you can replace the fader.

VERIFY the pot is bad with an ohmmeter before dis-assembling however.

You MAY be in luck. It looks like the cross fader has a connector which MIGHT allow you to remove it or certainly only a small board for replacement, seeing these take abuse.

It appears that the cross fader is a dual 100K type B taper.

The sliders on MOST of the mixer devices are pretty weak. They are subject to dirt or liquid getting into the slot and spoiling them. Fix is to replace them. You usually can find replacements at Digikey or Mouser that are suitable. The parameters are the resistance, the packaing and the length of throw AND importantly the "taper" of the resistance element.

how tow format pa800?

I like the Pioneer DJM-600