long in on the character to whom the dimension belongs.
Open your dimension tab by using [ or the menu (social, dimensions)
On the top of the right window box you get a large print of where the dimension is, and a smaller print of where the dimension is. The pencil icon (tiny and faded) is the key. The large is the one you can change. Click anywhere on the words, select them all and type what you'd like there.
This is also where you can change which dimension is active (if you choose a different one than the one that is currently in use, you will get a message telling you all your items will be packed away) **you cannot reopen a previous dimension with the same layout - everything was lost when it was changed**
On the second tab is permissions, where you can set your dimension to private, public view, guild view or friend view. You can also give any of those catagories option to move or place items in your dimension, but be careful, because if someone picks up an item in your dimension, the ownership transfers to them, whether you physically trade it to them or not. If they put it back down, the ownership transfers back to you.