replace broken Anderson S-400n DH Tilt wash assist?

Thanks for reaching out to us! I won't lie, installing this part can be very difficult. 

As for reinstalling, you have to compress its sides with your fingers and put it in pointed end first. You have to roll it in the same as you rolled the old one out.

It may sound easy on paper but it is a pain in the neck. If you really have a hard time with it, you may want the help of a local professional who has worked on Andersen windows like this before.