This article is from the Amiga FAQ, by Ignaz Kellerer with numerous contributions by others.
First let's drop some words on the Kickstart's Copyright: This belongs to Commodore, you *must not* use Kickstarts, without the right to do it! Especially it isn't allowed to make an image of anyone else's Kickstart and run this on your own Amiga. (I even doubt that it is allowed to do this on your own Amigas, if you have more than one.)
But of course it is possible and allowed for some people, developers for example. There are two different ways, a hardware solution and a software solution. The former is to buy a card which can hold two or more Kickstart ROMS and allows to select between when the System is booting.
The software solution needs a program (softkicker) and an image of the ROM. The softkicker allocates RAM, loads the ROM image into the allocated memory and reboots. Of course you have less RAM after the Reboot: 256Kb when running Kickstart 1.2 or 1.3 and 512Kb for Kickstart