I was added as a team member to another business account and when I accepted it somehow merged my existing business account ... How do I unwind this?
You can't. Only the team admin for the Business account can do it.
First, it's worth noting that one Business account can't join another, so you likely has a Plus or Professional account. Joining a Business account with a Plus or Professional personal account will merge the account into the team and refund any unused time on your subscription.
If you want to get your account back to how it was before, you need to contact the team admin and ask them to convert your account back to an individual account. It's very important that they don't just remove you from the team as doing so would delete your account. If that were to happen, the team admin only has seven days to restore your account.
Once your account is converted back to an individual account, you can resubscribe to the plan you were on previously.