Connect PC to USB port on printer
Download and run “Pronterface”
In Command Window enter M503 (report settings)
Look for a line starting with M92 and at the end you’ll see your current Exxx.xx value.
example. echo: M92 X80.00 Y100.00 Z400.00 E405.00

Send: G1 E100 F100 (extrude 100mm of filament)

Calculations here

M92 E <step value> (sets the new step value)
M500 (to save)
M501 (use new values)