This version of Desktop Hacker has a built-in assembler to allow you to alter instructions. Click Select on the disassembler window in the same way as for altering words, but on the right-hand side of the window where the disassembled instructions go.
Alter the instruction in the window that appears and click 'OK' to make the change. Note that simply clicking 'OK' without changing the instruction may change the word at that address, because more than one word can disassemble to the same instruction. This is not a problem as long as you stick to assembling real code.
The assembler is slightly more primitive than the disassembler, and does not support the new instructions introduced in the ARM3 and later. (These are rarely used under RISC OS.) Neither does it support AdrL instructions, which take up two words. But other than that it's okay really.
Using a cheat module
Putting the cheats into action