1. What AMD cpu do you want to install ?
2. What DELL PC model is it ? Do you have any data about its motherboard (brand/model)?
First you need to figure out what CPUs does the PC actually support, then theres a really small chance than if the CPU was originally supported on that DELL PC (that means it was one of the options available when you choose the configuration of the PC prior to buy it) that it will work, but and also as long as the BIOS is compatible. BIOS updates from DELL website are usually to add features or fix issues, but not to add CPU support.
Cheers