chrome keeps defaulting to yahoo

Hi guys

Really sorry if this has been discussed before but I cant seem to find a straight answer or anti adware software that works with my OS apart from something called Macwarrior that wants $50 to clean my hard drive.

I'm on a late 2013 mac pro running 10.6.8.

I have tried disabling all other search engines but everytime I start Chrome it defaults to yahoo.

Shall I install Yosemite?

Any advice would be greatly appreciated. Again apologies if this has been discussed at length.

cheers

HM