What you should do is find some flat ground and lay the tire down. After doing so you will need to break the bead. Breaking the bead requires a lot of force. My method is to run the tire over with another car. You should be extremely careful not to damage the rim.
Step 2: Remove the old valve stem.
Once you have gained access to the old valve stem you can remove it by cutting it or pulling it out with pliers.
Step 3: Lubrication/ glue
Next you have to use something will help the valve stem slide in place and when it dries you it should function like glue to prevent air form leaking out. I chose to use dish detergant but any other type of soap would do.
Step 4: Installing the valve stem
The installation process is straight forward. All you need to do is to put the stem trough the hoe and pull it trough. For these step you can use pliers or a valve tool. I use pliers because the tool was hitting the rim and I could not screw it in. I would recommend that you have a metal valve stem extender so you do not run into this problem.