Question: change of varibales

Hi,   I want to substitute every  U[jj+1,-1] by U[jj+1,1] in these lines. Many thanks to send your remarks.
U[jj+1,-1]:=U[jj+1,1];

for jj from 1 to M do:
  sys[jj] := eval(BTCS_general,j=jj);
od;

subs(U[jj,M+2]=U[jj,M]; U[jj+1,-1]=U[jj+1,1], sys);

  Why in this equation U[jj+1,-1] doesnt changed by U[jj+1,1];
                    s u[i + 1, -1]   4 s u[i + 1, 0]   6 s u[i + 1, 1]
      u[i + 1, 1] + -------------- - --------------- + ---------------
                           4                4                 4       
                          h                h                 h        

           4 s u[i + 1, 2]   s u[i + 1, 3]          
         - --------------- + ------------- = u[i, 1]
                  4                4                
                 h                h