Auction Bid Increment

Hi, I have been looking in vain to see if the answer has already been posted and I am aware that this has been asked a number of times over the last few years. I need to change the Bid Increment from £1 to say £10. I believe that the code for this is in defaults_auction.php, I have backed my file up, made what I believe are the modifications to increment by £10 but it doesn’t seem to work. Can you please advise if this is possible and am I in the right place? Also when you make a change to .php code does it work straight away once saved back to the server or do you have to restart something for the changes to take effect? Apologies in advance as I’m am by no means an expert but can generally get around code, any help on this would be greatly appreciated, Regards.