If you want to change the yield strength (fy) of a steelmaterial, you’ll have firstly to create a user defined material. In order to dothis, go on the Menu bar and click on Edit > Used Materials. In theMaterials window click on the “Add’ button, enter the name of the new materialand make sure that it’s family is set to “Steel”. After doing this, you canadjust the yield strength value (fy) from the steel properties button (see theattached image, 1.png).
Having this material defined, you can easily change thematerial for a planar element from its properties window (see the picture2.png).