We use google, here. You can search for stuff like ‘how do I reverse axis on a 3018’. Its really useful.
For example, if you want to invert the X and Z axes , you’d send $2=5 to Grbl and the setting should now read $2=5 (step port invert mask:00000101). This setting inverts the direction signal for each axis .
https://bitraf.no/w/images/4/4c/GRBL_software_Instructions_CNC3-3018Pro.pdf