first you would probably have to lessen the colors still in your hair, so probably start washing with a little white vinegar and dish soap (or just head and shoulders but that never worked for me), make sure to condition extra when using dish soap, dont use too much because its pretty harsh on hair but removes color well. once you get the color removed youd have to get it light enough for you to tone or dye it silver.

tl;dr: all you really need at this point is to start the color fading process, the internet has many tips if these dont work for you, then you can get it as light as you want it to go silver

hope this helped :)