The very simple block at the top of each page which contains the telephone and contact details can not be easily edited using the context edit menu. This is because the sticky (Affix-ed) navbar containing the main menu hovers over the context menu, so you can not click on it with the mouse, or use the arrow keys to select the edit item.
Instead you must go to Structure > Block layout > Custom block library where you will see a list of custom blocks. Here you will see the Tel & Email block. Edit its contents by clicking on the either the title or the Edit button on the same row.
Or just click here Edit 'Tel & Email' block
Note: This simple block contains embedded .svg icons which are not currently supported by the CKEDitor. It should therefore be edited in Source mode.