Insert in Visual Mode

Remember that, to be able to actually edit files loaded in radare2, you have to start it with -w option. Otherwise a file is opened in read-only mode.

The cursor mode allows you to manipulate with data at nibble-level (4 bits chunks), like it is done in most of hexadecimal editors. Press TAB to switch between hexadecimal and ASCII columns of the hexadecimal dump view. After you press i key, you are prompted for an hexpair string. If a is pressed, enter an assembler expression, which will be translated to machine code and then inseters at the chosen offset.