Is it compatible?

[cherry20]

If I want to have a different processor and memory do I just need to buy a new motherboard and make sure the RAM and cpu are compatible with it. https://support.hp.com/gb-en/document/c04738714 (My computer)

 

[USAFRet]

If I want to have a different processor and memory do I just need to buy a new motherboard and make sure the RAM and cpu are compatible with it. https://support.hp.com/gb-en/document/c04738714 (My computer)

For an All In One...you need to buy a whole new system.

Besides maybe the RAM and the hard drive, parts in that are not changeable.
You can't just slap in a new motherboard and be on your way.

 

[cherry20]

Oh I thought it would be less complicated than that. It seems I will have to build one after all.

 

[USAFRet]

Oh I thought it would be less complicated than that. It seems I will have to build one after all.

All In One's are sort of laptops in a different configuration.
Parts not interchangeable.

 

[jojesa]

If I want to have a different processor and memory do I just need to buy a new motherboard and make sure the RAM and cpu are compatible with it. https://support.hp.com/gb-en/document/c04738714 (My computer)

That's a proprietary motherboard, so you won't be able to get a replacement upgrade.
You could upgrade the RAM, CPU (35W) and HDD
Which CPU do you have?

I would upgrade to a SSD and 8GB RAM (if you have 4GB)

These are CPUs upgrades supported on HP Pavilion 23-q003na with a Crusher-U motherboard
Intel Core i5-4460T (Haswell) 1.9 GHz Quad Core
Intel Core i5-4590T (Haswell) 2.0 GHz Quad Core
Intel Core i7-4785T (Haswell) 2.2 GHz Quad Core.