After creating an element, the material can be modified in the properties window. If all the beams are selected and the material field is editable, then the modification is applied to all the beams. The same thing happens if, instead of beams, all the plates are selected.
In a regular model not all the beams and plates can be edited because these elements are created by joints and the material is taken from libraries.
To establish which elements can be modified, open the Model browser. To insert a new column, right click on the table header and choose one of the properties to display, in this case, material.
By clicking on Material, the properties from the table lines are rearranged according to this selection.
Note: all the properties in grey cannot be edited while the ones in black can be modified.
To modify the material for one element, the "Edit properties" option should be selected. To modify the properties for several elements, select the lines using the key for selecting successive lines or the key to select random lines, then right click on the model and access Advance Properties.