How do you access the RAM slots? From the picture on internal storage installation, I notice that in the middle there is a big black 2.5” hard drive mount. Do we need to remove it to access the RAM slots?
Yes you do.
Be sure you actually need the extra RAM – Danny’s estimate above is accurate. If you don’t need it it will literally be unused and wasted.
Here’s my NUC6i5 on Arch running my ~30K tracks collection (it rarely exceeds 1GB):
The magic of Linux/Unix
I admit I do not need 
as I have a similar library size as yours.
Currently I am running ROCK in an NUC 6i5 with 2x8GB RAM. The RAMs were cheaper then when I built it 2 years ago. I could hear the improvement when switching from running Roon Core in Windows 10 to ROCK when it was made available. But I always had this thought in my head that I probably could further improve the sound quality. Though 6i5 is quiet, it is not fan-less, any vibration when the fan kicks in will definitely degrade the sound reproduction. And correct me if I am wrong, I thought I read somewhere that the Roon OS in Nucleus+ is different from the free ROCK I am currently running in my NUC. With a fan-less Nucleus+ and its optimised Roon OS in Nucleus+, I could possibly extract a better sound quality from Roon.
My head and my heart are still debating if I should order a Nucleus+. If I do, I can just transfer the 2x8GB RAM (DDR4 2133 MHz) over from my NUC and future-proof the Nucleus+ for the foreseeable future.
OT but really? Where did you get this idea from? Roon uses RAAT which is bit perfect data transmission, end to end. How is a tiny amount of vibration supposed to influence this?
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Of course you can. It is a standard Intel NUC motherboard, I think they are using the 7th gen motherboard.
Something like this should work,
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Thanks for the info, just installed a pair of these:
Booted up and Roon was running after about 5 minutes
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Does the NUC need any BIOS config to recognise the new amount or is it just automatically detected?
Automatically detected
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Most every modern motherboard auto senses both ram and disk drives
Installed fully compatible memory 16GB, also replaced with a compatible faster M2 SSD 250GB. Result-increased speed, stability. 
Autoselection of all compatible components: 
how did you do that? did you clone the disk?
It’s simple. M2 was changed in the Roon, I just installed the memory.