Yes you change the ground textures form the Estate tools under the ground textures tab, and you can edit the terrain by right clicking the ground and selecting edit terrain form the menu. Then picking one of the many options. If you want to flatten the whole island/sim at one time. You can do so by first selecting the lower option. Do not pick a size leave it as it happens to be. Then click the apply to selection button many times. Until the land is under water and its all flattened to 0 meters. After doing that pick the raise option and push the apply to selection button until its the height desired

In order to divide your land you will need to right click on the ground then select edit terrain form the menu. Then left click on the ground, you will see yellow lines in the shape of a square. Hold the left mouse button and drag the lings as big as you want the land to be divided.

When you have estate powers you will have access to the Estate tools. To find the Estate tools you will need to click the world button at the top of your screen, then click the region/estate button near the bottom on the menu. With the Estate tools, you can restart the island/sim change the ground textures, set the agent limit (how many people come into the island at one time) set the water height restrict access to the island and many other things

Lets start off by restarting the island if its lagging. You do this form the estate tools debug menu. After pushing the restart region button it will take 2 minutes for the island to go offline. Then about 2 minutes for it to come back online sometimes a minute or 2 longer. This should take care of any lag issues you had. If not next step is to check your top scripts list in the debug menu for scripts using excessive Ms