The insert mode allows you to write bytes at nibble-level like most common hexadecimal editors. In this mode you can press '<tab>' to switch between the hexa and ascii columns of the hexadecimal dump.
To get back to the normal mode, just press '<tab>' to switch to the hexadecimal view and press 'q'. (NOTE: if you press 'q' in the ascii view...it will insert a 'q' instead of quit this mode)
There are other keys for inserting and writing data in visual mode. Basically by pressing 'w' key you'll be prompted for an hexpair string or use 'a' for writing assembly where the cursor points.