Google search switching results to Yahoo?

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

As you know there is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is desshoigned solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Also, unless you're using a true VPN tunnel, such as between you and your employer's or bank's servers, they are useless from a privacy standpoint: Public VPN's are anything but private.  

You should uninstall Avast according to the developer's instructions.

Appears that your Mac may have some form of malware that is redirecting Google searches to Yahoo. Have you tried using something like Malwarebytes for Mac to try to discover any yet? I'm only asking as we have no idea what these techs did for you.

Download and run the free version of Malwarebytes. It was developed by a long time contributor to these forums and a highly respected member of the computer security community. 

installing avast was a big mistake. the only malware removal tool that is commonly and rightly recommended in these pages is malwarebytes. (linked to by Old Toad above) many of us would never install avast or just about any other so-called "clean-up" tools. avast will likely just add to your problems. you should first uninstall avast per the developer's instructions. then use the link for malwarebytes to install it and find the malware that's causing the redirect in your browser. and IMHO, your 2015 is not "too old". it can run the current macOS, which even if this will be the last upgrade for that machine, it will likely be supported for at least a couple more years. no need to replace it yet unless you want to. :)