To edit value lists you need to be authorised for the CRM Settings. Open the menu at your profile at the top right:
Here you choose 'CRM Settings'
On the left, choose the section whose value list you want to edit, for example 'Relations' if you want to add an organisation characteristic. In the 'Value lists' widget, choose the relevant value list.
A value can then be added, edited, set to inactive or deleted entirely. If you set a value to inactive, it can still be searched for, but it can no longer be newly selected.