If you are using a hardware UART, you'll have to invert in hardware. You can either use one section of an inverter IC, like a 7404, or a simple one-transistor inverter . Both require regulated 5 volts. Just send it serially at the appropriate baud rate. To help the Backpack distinguish instructions from ordinary text, its program looks for an 'instruction prefix' in the data you send it.
Instructions correspond to the standard LCD instruction codes, and are listed in the manual.
The Backpack does not recognize these. Sticking to the LCD's own instruction codes lets the Backpack do very little processing of the incoming serial data. Of course, the larger the LCD, the more helpful terminal emulation is, so our 4x20, 4x40, and graphics LCDs do understand control codes. But they are still designed for power efficiency. We also have a growing collection of application notes .