keytool commands to replace existing SSL certificate?

I've a linux centos server running glassfish 3.1.2 app server. The default certs for ports 4848 and 8181 are 1024 bits. I need to replace these with 2048 bits versions. Looking for help to create the keytool command line code that does this.

I located the certs here:

# keytool -list -keystore keystore.jks
   Keystore type: JKS
   Keystore provider: SUN
   Your keystore contains 2 entries
   glassfish-instance, Feb 7, 2012, PrivateKeyEntry, 
   Certificate fingerprint (SHA1): 40:...:46
   s1as, Feb 7, 2012, PrivateKeyEntry, 
   Certificate fingerprint (SHA1): 3C:...:FC