How to install ram in it

From HP site

Maximum = Upgradeable to 32768 MB with optional 16384 MB SODIMMs in 2 slots

google this "how to install ram on a HP ZBook 15u G3" for a number of videos

@abdulbari12032 use the HP ZBook 15u Maintenance And Service Guide to work on your computer. Page 44 will tell you the type of memory.

4-GB (2133-MHz, DDR4) for computers with Intel processors HP part number 820569-001

8-GB (2133-MHz, DDR4) for computers with Intel processors HP part number 820570-001

16-GB (2133-MHz, DDR4) for mobile workstation HP part number 820571-001

and the amount. It will show you how to change it and how to prepare for it.

While you work on your laptop, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!