A group will display within the 'Personalise Accounts' section, under the Table options dropdown on the main cash flow page and can be removed or edited from there.

To rename the group, click the gear icon on the group name to access group settings:

To delete the group entirely, hit the trash can symbol shown towards the right of the row. This will remove the group and unbundle any accounts contained in the group. The accounts will revert to showing individually in the cash flow table.
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If you're looking to edit or remove an actual account rather than the group, click here.