Having been through this exercise myself, here's what you need to do:
First, before you do anything else, contact the manufacturer to ascertain what's involved. The manufacturer will tell you one of three things:
It can't be done. Although it's getting rarer, there are some components/makes which simply cannot be switched, they were built for a specific market and that's it.
It's a simple operation. More equipment now has a switch on the back of the unit where the voltage can be changed. However, as you're asking the question, I assume yours doesn't. However, just because there's no external switch doesn't mean it might not be a very simple operation - it could actually be a case simply of opening up the case and sliding a couple of jumpers to different slots.
It's do-able, but more complicated - specifically that there's no jumper move that can be done, but your unit can still be "fixed" by installing a new power supply and cord. You might get a good idea if this is do-able if your particular models are sold in different countries i.e. they aren't only sold in the US. It isn't a particularly expensive either: I was quoted around £65 to have an amp done.
Note that in the second and third cases, by opening the case, you'll almost certainly invalidate your warranty, even if you have it done by an approved dealer. That said, I would recommend that, certainly in the case of (3), you have a dealer do it in any case. That way, you minimize the chance of anything going wrong, you'll know the new PSU is the "right one", and if something does go wrong, the dealer will probably help you out: even though such work usually implies a disclaimer of some sort, it'd be a pretty hard-hearted dealer who'd screw you over like that.
I have a Denon receiver that was sourced in USA and that I now use in UK. I use a power transformer to convert my mains voltage from 220V to 110V and plug the receiver into the transformer. It works perfectly well.
I have also used step down transformers with success. I now have a proceed avp2/amp5 which has theoretrically been set up for UK use, but still has some hum issues. TBH these do not bother me and the kit is just superb!
Be aware that not all kit is so easy - the manufacturer tries to protect his markets by installing curcuits to detect voltage (easy to counteract) and frequency (much more difficult) and if you only do half a job, it will not sound like you might want too.