As long as you have not applied the Array Modifier yet, you can just UV the bottom stair and it will be automatically copied to the rest of the stairs. If you have applied it, your best bet is to just delete the top stars and redo the Array modifier again. There's not really any way simpler than that.
you can something that you should probably have done anyway: use an array modifier for the steps. add an empty at the center point of your staircase and rotate it on the z axis as much as the difference in rotation from one step to the next. then take your bottom step and make it a unique object (select all vertices of the…