If the Arduino is hosting the web page then the response to the HTTP requests will come from your sketch. You can code it to look up whatever values you want for inclusion in the page, and it is up to you whether you store them in voltale memory and look them up from there, or store them persistently in the EEPROM memory.
Changes to that stored value would need to be performed by your sketch too, and it is up to you to decide where the changes are accepted from and implement the mechanism to receive the updates and update the stored configuration. For example, you could receive updated in the form of HTTP requests, or by command over the Arduino's USB serial port.