I trust you have the material set up to 6x4 material slots?
Once those are used up you just create new ones over the old materials. Your old materials are still going to be in your scene if you still use them.
You can reset all material slots at once with the Utilities/Reset Material Slots.
In the toolbar below the material slots, the first one on the left (get material) will allow you to see all the materials in your scene if you pick the scene radio button. You can drag and drop these materials over your material editor slots, load materials from other scenes and drag them into your current scene etc.
To edit an existing material thats no longer in a slot, just pick the material in your max viewport with the eye dropper (next to the material name field) or drag them from the get material dialogue.
It doesn't take long to get used to how it works, but can be quite confusing for people starting out.
Once those are used up you just create new ones over the old materials. Your old materials are still going to be in your scene if you still use them.
You can reset all material slots at once with the Utilities/Reset Material Slots.
In the toolbar below the material slots, the first one on the left (get material) will allow you to see all the materials in your scene if you pick the scene radio button. You can drag and drop these materials over your material editor slots, load materials from other scenes and drag them into your current scene etc.
To edit an existing material thats no longer in a slot, just pick the material in your max viewport with the eye dropper (next to the material name field) or drag them from the get material dialogue.
It doesn't take long to get used to how it works, but can be quite confusing for people starting out.