You have not mentioned your objective of simulating problem in ANSYS. do you need to perform excavation kind of analysis ..first of all define the problem so that your question is more clear. From the error i.e. ''The volume is meshed and can not be changed' i understood that you may be need to to excavation kind of problem. During the analysis if you want to change the density of mesh then you need to start fresh and change the mesh density from coarse to medium or fine. I suggest you to read ANSYS manual. You give more details about the type of analysis you need to perform so that i could give still better reply to you..all the best..