If you download a replacement .exe installation file from the internet, first disconnect from the net, uninstall the incumbent AV, then run the installation file to install the replacement. Reconnect to the internet, update the signature files and run a full scan.
Have a look around the Symantec site for uninstallers as Norton sometimes has a scrappy uninstall that leaves residue scattered about the system.
As to the "Need to buy" if you're a non profit home user, you may prefer to buy, but there are a raft of free alternatives click here used by forum members that will do the job so you don't "need" to spend anything .
If you're a business user then unfortunately you don't qualify for the free versions.