[GRASS-user] change the cell value of a geotiff

[maven apache]

Hi:
I have a geotiff now, and some cells have a not-expected value, so I want to
Interpolation them, and I notice that there is a command named r.fillnull,
however the cell to be changed is not null, so I want to :
1) change the special  cell value to null
2) use the r.fillnull to Interpolation.

The problem is I do not know how to do the step 1 with grass, anyone can do
me a favor?

Thanks.
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