I tried to replace the tstop parameter of the text from 120 to 80. I got a single line of text: tstop 80, losing the rest of the text. I used

sed -i -rne 's/(tstop)\s+\w+/\1 80/gip'

I want to change only the line tstop and keep the rest of text as it is.

Part of the text is:

[Grid]
X1-grid    1     -6.0  24 u 6.0
X2-grid    1     -24.   96 u 24.
X3-grid    1     -18.0  72 u 18.0
[Chombo Refinement]
Levels           4
Ref_ratio        2 2 2 2 2
Regrid_interval  2 2 2 2
Refine_thresh    0.3
Tag_buffer_size  3
Block_factor     8
Max_grid_size    64
Fill_ratio       0.75
[Time]
CFL              0.3
CFL_max_var      1.1
tstop            120
first_dt         1.e-5
[Solver]
Solver         tvdlf