General form:

alterparam paramname=pvalue
alterparam subname paramname=pvalue

Example (global, top level parameter):

.param npar = 5
...
alterparam npar = 7 $ change npar from 5 to 7
reset

Example (parameter in a subcircuit):

.subckt sname
.param subpar = 13
...
.ends
...
alterparam sname subpar = 11 $ change subpar from 13 to 11
reset

Alterparam operates on global parameters or on parameters in a subcircuit defined by the .param ... statement. A subsequent call to reset (17.5.55) is required for the parameter value change to become effective.