You can send M92 X80 Y80, which will change the setting until Marlin restarts. You can send M500 to save any settings you’ve changed to EEPROM, which will make it persistent across restarts.
You can do all of that from Repetier Host, or any other console (octoprint, cncjs, pronterface, etc.).
Some settings can’t be changed through GCode like that, but there are a lot of settings that can. I think the most common things people might change are: Max feedrates, Max accelerations, Stepper driver current. These are all in the Configuration.h file, and when you flash it, they go into a sort of “defaults” area of memory. If you send M502, the stuff from the defaults gets put into the EEPROM, restoring the factory settings. There’s a lot you can learn by trolling the gcode docs on Marlin’s website too.
Have a lot of fun. Please share what you make. I look forward to seeing it.