Short answer: RAM, disk and similar upgrades are requested by ticket. Price and stock depend on product / quote — this article does not invent amounts.
Steps
Note the current configuration from the service detail.
Describe the upgrade clearly (e.g. +32 GB RAM, extra NVMe).
Open a support ticket and include the service ID.
After approval and invoicing you will get a planned maintenance window.
Turkey (Türkçe) 
