You can not edit the testimonials section from the block manager, but you can directly change the contents and other things via Cpanel or FTP.
Step 1: Login FTP or Cpanel
Step 2: Go to whmcsDir/templates/hostx/includes/blocks
Step 3: Find testimonials.tpl file
Step 4: Take a backup of the file and edit it according to your requirements. For this, you need to have some coding level knowledge.