Is it possible to replace the CPU with another CPU without having to unsolder it and solder it again? This is good if there are better CPU processors out there as well as for RAM?
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On what machine?
For the phone. No.
The librem phone
Oh how come it is soldered to the motherboard rather than being allowable to be replaced like on a laptop/desktop/tower?
That would be a new feature for the next model. Like the user replacable battery (I’m still pushing the joke of three triple-A’s.)
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Can you give any example of a recent laptop where the CPU is not soldered.
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Oh I thought you could replace the cpu on laptops, anyways why is it soldered to the motherboard and not replacable?
Yes, it would be indeed good that future CPU keeps software and pin-to-pin compatibility with i.MX 8M Quad (17Ă—17mm) 
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Oh I thought you could replace the cpu on laptops, anyways why is it soldered to the motherboard and not replacable?
Because that’s how things are being done in the ARM world, and not only there. For more than a decade, every device that’s somewhat smaller had soldered-on CPUs—remember Netbooks? Soldered-on CPUs. Pretty much every somewhat modern tablet ever? Soldered-on CPUs. Same for PDAs and Smartphones: Soldered on CPUs or SoCs. Size is the main reason, and also, soldering is a more solid connection (no risk of socketing failures).
FWIW, I am not aware of a single ARM SoC (System-on-a-Chip, a bundle of CPU, GPU, Northbridge, Southbridge into one Chip) that’s available socketed.
That said, in theory Purism could make a phone with a upgradable CPU and RAM going forward by using a SOM (System-on-a-Module) approach, where SoC and RAM live on a tiny daughter board pin-connected to the motherboard. The ClockworkPi uConsole uses that approach—so it’s possible, but it makes devices rather large and complicated.
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Our dev kit uses the SoM approach (https://source.puri.sm/Librem5/dvk-mx8m-bsb). It’s BIG.
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What if it used risc-v processor, is this still the same issue?
There is none for mobile and probably not even in the next 5 years minimum.
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(Simple answer - applying to lots of recent hardware, not just the Librem 5) Too many pins. And, more specifically, too many pins for the area available (areal density).
If you tried to do this socketed (i.e. pins into a socket) the chances of successfully removing the old CPU and inserting the new CPU would be poor. Even with vastly simpler chips with vastly fewer pins and much lower pin density, it is possible to bugger it up when putting the new chip in.
Soldering is also slightly more compact.
Even if you could replace the CPU, as @Quarnero says, this wouldn’t achieve much unless the replacement CPU is pin compatible and physically compatible (and supported by the software). Right now there isn’t anything to replace the CPU with so no actual benefit would be derived from making the CPU replaceable. But, sure, maybe in the future. In theory.
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The N class HP3000 PA-8600 processors were not soldered. But then again they had a 5 inch tall heat sink. There was room for 8 depending on your license.
It’s the “battle for thin” — soldering takes less vertical space than socketing. Not only that, on any machine that’s based on a SoM, it’s nearly as cheap to replace the motherboard (i.e. the cost of the motherboard is almost the same cost as the SoM). Of course, while Pine64 has a history
of selling upgraded motherboards, Purism doesn’t really do that so far (it’s probably 1/2 the cost of the device anyway).
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