You mean the vinyl covering, handles, etc?
I use a piece of cleaning cloth, wet and then squeezed dry (it's still humid but not dripping at all) to which I add a drop of dishwashing or window cleaning detergent, to wipe covering, handles, knobs, front panel, etc.; so as to pull dirt , fingerprints, etc. , then wash the cloth in clean water, squeeze it dry again and wipe again to remove any trace of detergent.
You can finish with a clean dry cloth.
After it dries naturally (1 hour) you can spray a little siliconen and wipe it with a new clean lint free cloth.
A common mistake many do is to skip the "washing" step and just spray silicone, it becomes a grimey paste with earlier dirt.
If the amp is old and dirt has encrusted into the Tolex, you can wash it out with a toothbrush and a little water/detergent.
It ends up looking new.