If you still have your old computer available, then the first thing to do is uninstall the software while connected to the internet. This will free up an install seat of which there are two.
If you don't have that option then you will need to go to your RL member page and deactivate the software.
You should also copy all content to an external drive as that can simply be copied over.
That done, you can reinstall the software and any resource pack and then copy over the content.
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