Fuel mixture

[Gajolo]

Hi Guys, I'm in Bristol, England, and new to the site and biking. I've had my bike since December, I've just changed the plugs and as well as being very rusty, they show that the air/fuel mixture is very rich and I get backfiring when slowing down , but doesn't miss when riding or accelerating. How do I adjust?? Oh and I'm not very good at mechanics
Any help would be appreciated