"You can’t upgrade RAM yourself but you can take it to an Apple Store or certified service center and they can upgrade for you," Ritchie recently tweeted.
When asked if this was confirmed by Apple, he replied, "Yes. Any service center, Apple or indie, can upgrade the RAM on iMac Pro post-purchase."
The new iMac Pro comes with a base 32GB of RAM. An upgrade to 128GB of RAM costs a whopping $2400USD. If you don't need the extra RAM immediately, you may want to consider pushing this cost down the road. Additionally, your service provider may be willing to source some cheaper RAM.
Disassembling the iMac Pro to upgrade the RAM yourself is likely to void your warranty.