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1. Click on the system tab within the side menu
2. Click on the Tab Edit option with in the Side Menu
3. Select the Permission level to be edited (See list above)
4. Click on the Search Tabs Button
5. This will return a list of all Tabs that meet the searched criteria
6. To reduce the returned options in this box  above the returned results start typing the name of the required tab. This will reduce the number of results. 
7. Click on the blue hyper link on the left side of the required record.
8. Click on the pencil icon next to the option to be edited
9. Update the fields to be edited (Please note: Only label, Order number and Visibility can be edited)
10. Click on the Save Tab Button. 

Further reading

How to Search for a Side Menu option

How to edit the Side menu option

Permission Levels