To set the eax register on the x86 processor cpu0 to 3, the command invocation is write-reg cpu0 eax 3. There is also a namespace version cpu0.write-reg eax 3 or the more convenient variant %eax = 3 that writes a register on the current frontend processor.
This function may or may not have the correct side-effects, depending on target and register. If no cpu-name is given, the current frontend processor is used.