[Courtesy of Gizmo's Freeware]
It's always a good idea to change your important passwords regularly. By doing so, you ensure that someone who may have previous discovered your password can no longer continue using it. Also, if someone previously discovered your password but has not started using it yet, they won't be able to start.
If you use a cloud storage account like One Drive, Google Drive or Dropbox, this definitely counts as one of your important passwords so you need to change it regularly. Even if you really don't like the bother of changing passwords, doing so at least once a year will suffice.
Back in 2012, there was a rumour that Dropbox had been hacked and that a few million usernames and passwords had been taken. Recently, Dropbox has confirmed that some of those stolen passwords are now circulating on the internet. If you haven't changed your Dropbox password since 2012, you'll be automatically prompted to do so by Dropbox. But even if your password is under 4 years old, it still makes sense to change it.
So if you're a Dropbox user, change password. Note that you only have to do this once, from one device or computer. All the other devices that are currently connected to your Dropbox account will still work, and continue to sync their files.