Hi lenhalio,
Welcome to EasyEDA.
You can't change the *default* dimensions to mm instead of mils but you can set them for a login session.
Open the right hand **Properties** panel (click on the crosshatched vertical blue bar at the right hand edge of the window).
Under **Canvas Attributes** find **Units** then, from the drop down menu, select **mm**.
Note that this also sets the DRC (Design Rule Check) units:
**Super Menu > Miscellaneous > Design Rule Check...**
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