There are 3 fixes for this: the simple way with software, the harder way with software, or the PITA way with hardware…
Simple method (my preferred method): redo the machine setup in Easel (Go to Machine>Setup New Machine), during the process there is a page with a grid and some yes or no questions regarding the operation of each axis, this step is commonly skipped over, BUT by answering these questions properly Easel will automatically correct the grbl setting that controls the direction of travel for you.
The harder (manual) software method:
you can manually do what Easel does in the simple method by opening the Machine Inspector page in Easel (press ctrl+Shift+D to open this page, or go to Machine>General Settings>Machine Inspector to navigate there) and scroll to the bottom of the page where you’ll find the grbl settings. Read the value for $3 and then plot the current setting on the chart below. Then select the new value that changes the axises that you need changed. Then scroll back to the top and in the “console” section type the new setting value just like you want it to appear in the setting list below and hit enter to send the command to the cnc where it will be saved until updated in the future…
for example IF your current setting is $3=1 and you need to chance Y and X . the new value to use is $3=2 because i kept the Z setting as it was, a “N” value, and I altered the values of both X and Y respectively.
The 3rd and final method is to re-wire the stepper motors to reverse the direction.
**AND IF your problem was the Y axis only and you’ve got an X-Carve or other Belt driven CNC, this would actually be the simplest for reversing the Y only because you could swap Y1 and Y2 and that will invert the movement of the Y, but since you need to also swap the X, the Easel setup is by far the fastest/easiest fix.
Merry Christmas!