Reduce a potentiometer value from 1M to 500K
As we don't have a schematic, I'll take a guess.
Looks like a simple triac-diac &quot;dimmer&quot; type circuit. In such a case, the potentiometer is connected as a rheostat. In such a case, the center and the left pin (from the front, pins down) of the pot are either connected to each other or the left pin is unconnected.
So you can try the 1M pot with 1M 1/4-W resistor in parallel, connected as the original. The linearity will be affected but it should not be exceptionally worse than it is.
As always, keep in mind that working with mains voltage is inherently hazardous and be sure to get help or abandon  the project if you are not thoroughly familiar with appropriate safety measures.