When a user is deactivated, the companies they manage keep them as account manager. To hand those companies over, filter the company list by the user who left and update the account manager on all of them at once.

1. Go to Companies and open the Account manager filter.
2. Select the user who has left. If they are not in the list, their user was deactivated with Visible in dropdowns and filters turned off. An admin can turn it on under Settings → Account → Users & roles (click Include inactive to find the user, then edit them). The user stays inactive and can't log in.
3. Select all companies in the list and choose Update companies.
4. Under Account manager, pick the new account manager and click Save action. If your companies can have more than one account manager, choose Set as sole account manager to replace all current account managers. To keep the others, choose Add for the new account manager, then run Update companies again with Remove for the user who left.

To find companies that have no account manager at all, select No account managers in the same filter.