The Intel chipset is not upgradeable to an 8th gen CPU.  It would require a different motherboard and given all the Dell proprietary items that means also a new case and power supply - a new computer. Your call on a new PC now or later.  
CPU fitted is Intel(R) Core(TM) i3-7100 @ 3.90Ghz, 3912Mhz, 2 cores, 4 logical processors. As this is considered incompatible with Windows 11 I would like to know what compatible processor is recommended and can be retro-fitted, and are there any other parts that will need replacing (Motherboard / Chipset etc:). Is the expense worth it or should I let it run Windows 10 for four years and then buy a new computer.  
Inspiron 3268 CPU Upgrade for Windows 11