The firmware can be changed at any time using a PicKit 2 or PicKit 3 ICSP programmer.
Plug your PicKit 3 into a spare USB port, then start MPLAB. Once it has loaded, click Configure > Select Device to bring up the device selection window. From the device drop-down list, select PIC16F1938 and click OK.
A dialog saying "New firmware must be downloaded for PicKit 3 to work with the part selected. Click OK to continue." may pop up at this stage. If it does, click OK and wait for MPLAB to download the programming firmware to the Pickit 3. You will see some activity in the PicKit 3 tab of the output window for up to a minute or so. When MPLAB is ready you will see "PicKit 3 Connected" and possibly an error stating "PK3Err0045: You must connect to a target device to use Pickit 3". This is not a problem and just a warning that the PicKit 3 cannot "see" the chip in the APC819
Then, click File > Import and browse to the 819_1v0.HEX file (latest version at time of writing) and click Open. The last line of the build tab of the output window should read "Loaded C:\...\819_1v0.HEX." which indicates that the hex file has been loaded into memory.
Now, connect the PicKit 3 the 6 pin ICSP header next to the micro. Make sure the arrows on the Pickit 3 and the 6 pin header are aligned. Then power up the APC819. The PicKit 3 tab of the output window should now read "Target Detected, Device ID Revision = 50900053" or similar.
Click Programmer > Program to initiate the ICSP operation. The PicKit 3 tab of the output window will show "Programming...", then "Programming/Verify complete" once it is done. Disconnect the PicKit 3 from the APC819. The PicKit 3 tab of the output window will show "Target Removed".