Help is quickly at hand here: have the blades of the lawnmower sharpened or buy new blades if the old ones are beyond repair. For re-sharpening, you should preferably give the lawnmower to an expert, for example at a D-I-Y store or specialist dealer. There the blades can be sharpened inexpensively.
However, if grass blades become yellow, this may also be a sign of iron and nitrogen deficiency. In such a case, liquid iron fertiliser ensures a beautiful, lush green lawn again within a few days.