The HP ENVY Ultrabook 4-1100sl uses a 4-cell, 52WHr 3.55AH Li-ion battery with HP Spare Part number 681949-001.
For replacement procedures, you do have the Maintenance and Service Guide, starting from page 43 of the manual, and you may also be able to find a relevant video on the HP Support YouTube channel. However, without the proper safety precautions and experience, computer part replacements can go terribly. Therefore, if you do not feel confident, I highly recommend you find a technician at a local computer repair shop.
Furthermore, you can find parts through the HP Parts Store. You'll find the majority of parts available there but it's common for them to be back-ordered or unavailable. You may be able to find the part online through third-parties.