I have been given pen assignment in a .stb format but I need it on a .ctb format. I am working on Autocad release 2015 light thank you  

The Mad Cadder wrote this...To convert a STB to a CTB:
1. Open the STB based drawing
2. On the command line type the variable: CONVERTPSTYLES
3. Follow the on-screen instructions.

Thank you for your help. but it didn't work! this is what I did; I putted the .stb inside autocad plot style folder, them I close and open the drawing, then I type convert styles follow the instruction but I couldn't find it in the plot style browser. So, I got stuck!  

Did you type CONVERTSTYLES or CONVERTPSTYLES?  There is a difference.And why would you have to put anything in the plot style folder in the first place?  Makes no sense.