You can select separate measurement units for LibreOffice Writer, LibreOffice Writer/Web, LibreOffice Calc, LibreOffice Impress and LibreOffice Draw documents.
Open a document of the type for which you want to change the measurement units.
Choose LibreOffice - PreferencesTools - Options.
In the left pane of the dialogue, double-click the application for which you want to select the measurement unit.
Double-click LibreOffice Writer if you want to select the measurement units for text documents.
Click on General.
On the General tab page, select the measurement unit. Close the dialogue with OK.
LibreOffice - PreferencesTools - Options - LibreOffice Writer - General