If you are not comfortable releasing your legal claims, or with the amount offered to you in exchange for that release, you basically have two options (assuming you don't want to sign the agreement anyway):
Let the deadline expire and consider filing a lawsuit.
Attempt to negotiate a better package in exchange for the release.
If you choose the first option, you need to know your deadlines for bringing a lawsuit. If your claims relate to discrimination, sexual harassment, or retaliation for reporting or objecting to either of those things, you only have 300 days (approximately 10 months) to bring a claim. For all others, the statute of limitations is either two or three years.
The second option is one many people choose. If your employer will not negotiate the package, you still have option number one available. In many cases, however, the window of time you are given to consider the severance is a good opportunity to explain your legal claims to the employer and ask for an increase in what they will pay you for the release.
If you choose the second option, you should seek advice from an experienced employment lawyer who can help frame and articulate those claims and negotiate with the employer.