You need to speak to them and actually "negotiate" it: you can't just write in different terms or amounts that you'd like. Before doing this:1) Bear in mind that if you negotiate, counteroffer, etc. the package, that is a rejection of their offer. They don't have to make it to you again or let you accept it: they could pull it off the table entirely and give you nothing (see below).2) If you are an at-will employee, you have NO right to any severance: the law does not require it. They could terminate you without providing any whatsoever. So you do not have any leverage to get more: whatever they offer is purely voluntary on their part.