I have the following statment that if isdefault is true to this collection i need to set each object isDefault property to false.
custHead.lstCustomziation.Whe
Linq is for querying, not updating. You can get a list of the items you want to change and then update using a normal loop:
var list = custHead.lstCustomziation.Where(x => x.IsDefaultSelected == true) foreach(var item in list) item.IsDefaultSelected=false;