The documentation for blupf90 is available separately as a PDF document.
The following lines can be added at the end of parameter file for blupf90:

OPTION conv_crit  1e-12
   set convergence criteria; deault is 1e-10

OPTION maxrounds  10000
   set maximum number of rounds (default is 1000)

OPTION solv_method  FSPAK
   selection solutions by FSPAK, SOR or PCG (default is PCG)

OPTION r_factor  1.6
   set relaxation factor for SOR (default is 1.4)