Code:tran1 tran stop=0.5u param=temp param_vec=[0 20 50n 25] param_step=0
which will have the temperature at 20 degrees until 50ns when it will change to 25. Or:
Code:tran1 tran stop=0.5u param=temp param_vec=[0 20 50n 25] param_step=10n
This will take 10ns steps between the temperature values at 0s and 50ns, so will rise by 1 degree every 10ns.
In ADE the param, param_vec, param_step fields do not appear (yet) on the tran analysis options form, so you'll need to type these parameters in the "Additional Parameters" field at the bottom of the tran options form.
This capability was added in MMSIM62, so you'll need to use a version of spectre which is MMSIM62 or later.
Generally I would advise against taking too big a temperature change in one go because this potentially could lead to convergence difficulties as you suddenly introduced a big discontinuity into the simulation (and of course, that would be completely unrealistic).
Best Regards,
Andrew.