> \> w? <br>
> wx[?][fs] 9090       write two intel nops (from wxfile or wxseek)

First seek to the address you want to write, then write:

    > s 0x000009d5
    > wx 34313335

You can also use:

    > w \x34\x31\x33\x35

This will write '4135' at the address pointed by `s` command

Radare2 also give you the option to point to the address that you want to write on:

    > w \x34\x31\x33\x35 @0x000007b0

Write "4135" at 0x000007b0

Use the one you feel more comfortable.