Okay, look on the left side. There should be the tileset you have on (Probably World, either upper or lower layer.)

Underneath that should be the name of your game, and under THAT should be the names of your map(s). Right-Click on the map you want to change, and select Map Properties. A new window will appear. On the top left part, there is the name of the map (change it if you want). Beneath that, there is the ChipSet selector.. select the set you want.

----------------

I'll bet, that since you didn't know how to switch the chipsets, that the "two" chipsets you mentioned are actually one and the same: There are two layers to each chipset, Upper and Lower. Lower is typically the general terrain (grass, water, etc) while Upper is for placing objects (trees, rocks.) Upper has priority over Lower.

For example: a tree is on a grass square. You could normally walk on the grass. However, since something from the Upper layer is there, and that particular object can't be walked over (unless you change that), you can't walk on that square.

WARNING: If you have a map made, you shouldn't change the chipset, or it will mess up the map. Quite simply, the tiles are switched. If the 5th tile is say, desert on one of them and on another the 5th tile is water, when you load the map in the new chipset, it reads the data as "5th Tile at location X,Y" and ignores what it used to be.

Understand all of that? If you need something else, just ask...

EDIT: Typo correction.