hp15-dw3048ne Can I upgrade the battery to a bigger one?

Unfortunately, it looks like that model only supports one type of battery; the HT03XL which has small variations from 40Wh-to 41.9Wh and share 3cells among the various third party manufacturers. Chassis' like this are generally designed with one size in mind, whereas older ones with user-replaceable batteries (that pop off the back) can be extended a bit.