If you have access to it (FTP or web interface) you'll want to change that setting in your server's
server.cfg, which is located in your servers csgo/cfg/ directory.
Whatever you put there will be permanent as it will be executed every map change.
However, if you want to customize more settings it's recommended to use a custom config file. Just upload it to /csgo/cfg/ and give it a proper name, like
1v1.cfg. Put all your custom settings there and leave them alone in the
server.cfg. Now add the line
exec 1v1.cfg (or whatever you named your custom config) to the bottom of your
server.cfg when you want to use those settings and remove it when you're done. Of course this also works with multiple different configs.