sheet1

column A column B
R1
R2 ABC 10
R3 DEF 12
R4 LMN

sheet2
column A column B
R1
R2 ABC 9
R3 XYZ 13
R4 DEF 11
R5 PQR 15


I've to find out all the column A values of sheet2 that are not in
column A of sheet1 (in this example XYZ and PQR) and paste the entire
row of sheet 2 in the next null row under column B of sheet1. Since
null row under column B of sheet 1 is R4, R4 will be overwritten.

Thus the output will look like

sheet1
column A column B
R1
R2 ABC 10
R3 DEF 12
R4 XYZ 13
R5 PQR 15