To Change the Default Lineweight Assigned to a Layer

Click Home tab Layers Panel Layer Properties.
In the Layer Properties Manager, under the Lineweight column, click the lineweight that you want to change.
In the Lineweight dialog box, choose the lineweight that you want to use as the default for objects on that layer.

Note:

The lineweight that you choose is used for all objects on the layer that have a ByLayer setting for their lineweight property.
Lineweights are not displayed automatically. If you want to display or hide lineweights, click the Show/Hide Lineweight button on the status bar. If no change is visible, it's probably due to a combination of the thickness of the line compared to the display resolution of your monitor.