In[]:=
Button["Set unit system to SI",userBaseDirectoryStream=OpenAppend[FileNameJoin[{$UserBaseDirectory,"Kernel","init.m"}]];WriteLine[userBaseDirectoryStream,"$UnitSystem=\"Metric\";"];Close[userBaseDirectoryStream];BaseDirectoryStream=OpenAppend[FileNameJoin[{$BaseDirectory,"Kernel","init.m"}]];WriteLine[BaseDirectoryStream,"$UnitSystem=\"Metric\";"];Close[BaseDirectoryStream];]
Out[]=