I can change inch-->mm in solidworks temporarily.
but I want change forever
1-open new part
2-click on tools(it is on the top,left your screen)
3-click on option
4-go to document properties(which is the beside of the system options
5-click on units
6-change your units as you need then click ok!!!
7-now click on files and then click save as!!!!
8-save as metric unit,just attention that change save type into *prt.*SLD
9-now click save and close the part
10-now open the new part and go to templates and open metric units
open new part
set units to metric in options tab
Save as metric.prtdot
New parts using metric.prtdot template
Delete old part template if desired
With this new part template you will be starting with metric units
Open new part , then
Tools -> Document Properties
Under left side choose "Units"
Under the Units tab choose MMGS (millimeter,gram,second) or custom units.