I have new 200 amp.service in my home;then I had a second box installed,and it is 100amp. and that also runs to my guest house,which is 100 amp.How do I change the first box to 20 amps. and then change the one in the 'guest house" to 200 amps.

You call an electrician, let him look at the job, then he'll tell you how much it's going to cost.

I'm just curious about your installation.

You have a 200 Amp service from the Power Company. Off that main service panelboard, you have a 100 amp panelboard inside your home? Also, you have an additional 100 amp panelboard at your guest house? Is that panelboard fed from the main service panelboard or the 100 amp panelboard?

And now you want to keep the 200 amp service at your home, decrease the second panelboard to 20 amps(?) and increase the guest house feed to 200 amps?

Stan is absolutely correct, you need a licensed electrician to do a load calculation and determine whether you need the Power company to increase your service or place the guest house on it's very own service.