I have installed qemu by slackbuild. Now I want to install newer version of it from official website. Is it necessary to remove the old one before install the new version? 
Should I remove previous version of software before install new one?
software installation;slackware
Since you're mixing a package that was provided by slackbuild with one that comes from the official website I'd be inclined to remove the previous package first. Then install the official one.Prior to doing the removal, I'd make a backup of any configuration files just so that you have them as a reference prior to doing the new install.