To change the Dropbox account your computer is linked to, unlink your computer from your account settings or desktop application preferences.
Unlinking your computer from your Dropbox account means Dropbox will no longer sync changes to the files and folders in your Dropbox folder from that computer, effectively turning off the Dropbox service. The files themselves remain untouched in your Dropbox folder.
To unlink your computer from your account:
At this point, your account is unlinked and Dropbox will prompt you to link your computer to another account or prompt you to create a new account. If you are linking your computer to another account, you may want to move your old Dropbox folder somewhere else first. Otherwise, when you link another account, Dropbox will ask if you want to merge its existing files with your new account.