Why am I not able to upgrade my system?

Solution 1:

Your output (Connection failed [IP: 91.189.88.142 80]) suggests a problem at the server end, not your end. You cannot fix it.

Interestingly, I'm seeing some of the same problems. So among the choices below, "wait" is a viable alternative for this case.

• For many users the simplest and quickest solutions is to try a different mirror.

• If that mirror has worked before, you can try simply waiting for the mirror to be fixed by the server's owner (or for access to be restored by the ISP). Mirrors do occasionally drop out briefly in order to sync. Occasionally the hosting servers come under Denial Of Service attacks. There are other temporary possibilities.

• If you are on an enterprise or school network that requires use of a proxy, contact your IT department for their proxy instructions.

• Internet Service Providers in some countries may restrict access to Ubuntu Mirrors due to mistakes or policy. Sometimes the restriction is for something else that's hosted on the same server. You can contact your ISP and ask them if they are restricting access to that IP address.