iStore Cresta

My macbook air was taken in to iStore Cresta and they could not do the software upgrade as on start up it has a prohibited sign and does not fully boot. I once took it to iStore Clearwater and they sorted the problem out 6 months ago by installing the operating(el capitan) system from an external drive.

Cresta however is unable to help and they did not even try to install elcaptain I spoke with Sihle who notified he will get back to me but I am yet to receive feedback this is bad service and he does not know how to manage customer expectation.

Please sort this out urgently.