In any wooden floor setup, one of the main components is the plywood subfloor. This acts as the base to the rest of the floor, and helps it maintain its integrity and still look pleasant over time. If you have your floor installed for long enough, the subfloor can begin to deteriorate. Moisture from the above world can seep in and cause rotting or molding.
The boards can also naturally warp over time, causing tiles of the wooden floor above to pop out of place or create small bumps. If your plywood subfloor has been acting up and you feel that life would be better if you could replace it, then rejoice. It is easy to replace the plywood subfloor, and with just an afternoon of work you will be able to have a floor like new.
First, you will need to remove the top layer of flooring that is visible at most times. You can take this up piece by piece, exactly how it was laid down. Set it aside in a large stack in another room, and you will have access to the subfloor. It usually will not look normal, particularly if you have had the floor laid down for a while.
The large pieces of plywood may be warped or rotted. Once you have them all ripped out, dispose of them properly. Removing them may be difficult, particularly if they have rotted to the point that they fall apart fairly easily. If this is the case, just get a trash can in there and use a shovel to hack it away. Be sure to clean it all out and make sure that the ground is level once you are done.
Once you have all of the plywood out of the ground, you are ready to replace it with new and fresh plywood (and hopefully take some measures against whatever factors caused the deterioration of the previous pieces). Take this opportunity to improve the materials by not replacing with luan pywood. You can also improve the conditions using many different sealants which you apply to the bare floor, and which will protect not only the plywood on the subfloor, but the above layers as well. (See Different Types of Plywood)
This will mean that you will not have to change it in the future. After you have taken these preventative measures, you will lay down new plywood to take the place of the old stuff that you ripped up. Depending on the size and shape of the room, you can layer the plywood in with minimal cutting. If cutting is required and you do not have the tool you need, you can rent it or borrow it from a friend.
Once all of this is finished, you can replace the top layer of floor as if it had never been removed. You will notice an immediate difference in the look of your room, depending on how bad the problem was before. If you had dealt with warped boards, you will immediately notice that your floor is flatter, and probably creaks less than you are used to.
So if you suspect that your subfloor has lost some of its original quality, then you should look into replacing it. It is very quick, and something that anyone can do. Just get a friend to help you carry the pieces of plywood, and go for it. It will improve your floor more than anything else you can do.