Here is the documentation. https://www.gnu.org/software/sed/manual/sed.pdf
PLease forgive me, I have not used it, but it is close syntax to DOS. Your scope of the question is not direct, you will have to figure out what this model is, and apply your problem to this model. I am not gong to give you the exact code to make it work, as you have asked for a particular line, and the example has it in there.
$ sed '2d a\--end--' input.txt > output.txt
This should be deleting line 2 and inserting --end--. Once again, not tested.