In Autocad, what is the command that is used to rotate the grid at 45 degrees?

To rotate the snap grid

1. Enter snapang on the command line.
2. Enter a value for the new angle. (The default value is 0.)

 The angle that you specify is relative to the current user coordinate system (UCS).

UCS order , you can disclose it from the AutoCAD bar

WRITE SNAPANG IN COMMAND AND ENTER THE NEW ANGLE 45 

https://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/Changing-angle-of-snap-grid.html

"To rotate the snap grid

Enter snapang on the command line. Enter a value for the new angle. (The default value is0.) For example, setting the value of the SNAPANG variable to will rotate the snap grid by degrees.

Note: The angle that you specify is relative to the current user coordinate system (UCS)."