To alter the value of an address's contents, click Select on the address in the disassembler (in the left bit, not on the disassembled instruction). The value in the box is the contents of the address in hexadecimal; if you want to know the hex version of a decimal value, go into BASIC and type PRINT ~number, where number is the decimal version.
Type the hex value and click on 'OK' to make the alteration.
Making a cheat module
Putting the cheats into action