how to fix a dented speaker cage?

Speaker cage? Perhaps you mean basket. How about some more details? If you dented the basket on a speaker driver there really is no way to fix it. If does not interfere with the performance, such as causing the voice coil to make contact wit the magnet structure, you needn't worry about it. If it does, junk it. It's not worth fixing. Expensive drivers have cast baskets that will break before they dent.

i'm not keeping attempt this, yet I once knew of a guy who may upload water to the paper tweeter dome and after it became quite moist, he may get a ingesting straw and suck it out. maximum folk may use protecting tape notwithstanding it continually leaves a imprint. strong success!