FPU.FSET(register 1...15);

example 1:
#include<LCD4_128.h>
#include<FPU_128.h>

void main()
{
   float x;
   
   Init_Fpu();    // Initialize FPU
   
   FPU.SELECTA(6);           // Select A as register 6
   FPU.FWRITEB(3,0.005); // Write 0.005 into register 3 and select B
   FPU.FSET(3);     // A = B  (B is register 3)
   
}