To change pressure, simply unfix the current fix and create a new one.

For shrinking, check:

http://lammps.sandia.gov/doc/fix_deform.html

Thanks Mr Marcel Balcik for your answer,I want now to know what change I can add on this program lammps to apply a deformation on a system on which I already applied a pressure of 20GPa here is the program

# ------------------------ INITIALIZATION ----------------------------

units metal

dimension 3

boundary p p p

atom_style atomic

read_data data.infopress

timestep 0.001

pair_style eam/alloy

pair_coeff * * Al.eam Al

# ------------------------- SETTINGS ---------------------------------

compute csym all centro/atom fcc

compute pe all pe/atom

compute temperature all temp

compute 2 all stress/atom temperature

compute P all pressure temperature

######################################

# EQUILIBRATION

reset_timestep 0

timestep 0.001

velocity all create 300 12345 mom yes rot no

fix 1 all npt temp 300 300 1 iso 0 0 1 drag 1

# Set thermo output

thermo 1000

thermo_style custom step lx ly lz press pxx pyy pzz pe temp

# Run for at least 10 picosecond (assuming 1 fs timestep)

run 20000

unfix 1

# Store final cell length for strain calculations

variable tmp equal "lx"

variable L0 equal ${tmp}

print "Initial Length, L0: ${L0}"

######################################

# DEFORMATION

reset_timestep 0

fix 1 all npt temp 300 300 1 y 0 0 1 z 0 0 1 drag 1

variable srate equal 1.0e10

variable srate1 equal "-v_srate / 1.0e12"

fix 2 all deform 1 x erate ${srate1} units box remap x

# Output strain and stress info to file

# for units metal, pressure is in [bars] = 100 [kPa] = 1/10000 [GPa]

# p2, p3, p4 are in GPa

variable strain equal "(lx - v_L0)/v_L0"

variable p1 equal "v_strain"

variable p2 equal "-pxx/10000"

variable p3 equal "lx"

dump 2 all custom 1000 dump.defo.* id type x y z c_csym c_2[1] c_2[2] c_2[3] c_2[4] c_2[5] c_2[6] c_pe

fix def_print all print 100 "${p1} ${p2} ${p3} " file mg001.defo.txt screen no

# Display thermo

thermo 1000

thermo_style custom step v_strain v_p2 temp v_p3 etotal press

run 50000