Best Products Co. of Richmond has reported its earnings increased in the second quarter and the six months ended Jan. 2, but said heavy response to sales late last year reduced profit margins.

For the quarter, Best earned $24 million ($2.47 a share) compared with $21.9 million ($2.26) for the same period last year. Sales for the second quarter were $495.5 million this year and $442.5 million last year.

For the half, Best posted earnings of $26.6 million ($2.74) on sales of $732 million compared with $26.2 million ($2.70) on sales of $629 million last year. All earnings have been restated for accounting changes to the last-in, first-out method.

Best also said it was going to change its fiscal year to end the Saturday closest to the end of January--Jan. 30 this year.

Richmond, Fredericksburg and Potomac Railroad Co. reported net income of $16.7 million ($46.35 a share) in 1981, compared with net income of $15.7 million ($43.84) in 1980. The company said operating revenues from its railroad operations totaled $53.9 million last year compared with $49.6 million in 1980.

Fair Lanes Inc. of Baltimore earned $1.7 million (39 cents a share) on sales of $21.5 million in its second quarter ended Dec. 31, compared with profits of $1.6 million (35 cents) and sales of $14 million in the quarter a year earlier.