You can upgrade the memory with no issues at all. The CPU I pretty much doubt it unless the BIOS revision accepts it.
The GPU can be upgraded as well but probably you wont go far as the PSU is proprietary too.
If you got a very very slim wallet you can go as far as 16Gb DDR 4 2133mhz and maybe a 5500XT, 6500XT, 6600 non xt, 1660 super or RTX3050 (without upsetting the PSU). Be careful, you may want to check if it fits inside the case because DELL cases are super thigh and sometimes they use low profile GPUs.
If you should do it?
NO!
Save the money for later and get a decent non proprietary system.
The Englishman