Hello Sarah, this is Emily.
Go to https://myapplications.microsoft.com, sign into your account.
Once in, on the upper left, click on the upside down arrow and click on My account.
Then on the left, click on "organization". Find the old job (organization), and click on Leave.
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I am not sure if you are using a personal Microsoft account or business account to sign into Teams. If you are using a personal account, you will need to switch to the new Teams free version after you removed yourself from the old job.
You can download it from here: https://www.microsoft.com/en-us/microsoft-teams/download-app#for-desktop Sroll down a bit and download the Teams for home or small business on the left.