In case you need to edit an account's data, you have two options how to proceed. If you only need to do minor changes, use inline editing. For complex changes, use the editing mode.
Inline Editing
To apply inline editing, double-click the data entry you wish to edit. The data entry now turns into a field you can edit. After editing, click Save to complete the editing process. The altered entries are now set in bold orange (Classic) or highlighted in yellow (Lightning). Or click Cancel to discard all changes.
To edit an account using the open editing mode, open the account record and click Edit.
Use of Editing Mode
You can now edit all the information on the account. When you are done, click Save.
Merge Accounts
To merge accounts you can use a Salesforce function which can be found at the bottom of the Accounts tab in the link list. More information can be found here: Salesforce Help on Merging Accounts.