Disclaimer: Have never actually used HEF so this is not based on experience but rather on how I believe/think it works.

Since HEF is part of FLASH I'd say the command(s) to use is ERASECODE/WRITECODE/READCODE. And since FLASH memory is sectioned into blocks or rows you need to take that into account if/when you're trying to overwrite/change something you've already written to HEF. In that situation you need to read out everything within that block/row, save it to a buffer (ie RAM), change the value in question, erase the block in HEF and then finally write the content of the buffer back to HEF. For the 16F15376 that you mention in the other thread each row is 32 words.

See section 13 of the 16F15376 datasheet for more details.