I want to overwrite my old xls file with the new xls file each time i generate a new data

I am new to matlab and I am using "xlswrite" operation. At 1st, I am generating 150 data into a xls file. After the 1st testing, I am thinking to generate 2 data into the xls but only the 1st and 2nd data in the xls file being overwritten and data from 3rd until 150th of the 1st testing is still there. What can I do to make sure I only get the numbers of data that I want in the xls file?

My codes as below

clc

a=input('Enter Number of Cities need to travel: ');
b=2;
city= randi(100,[a,b])
xlswrite('Data.xls',city

Answer 1

[LY Cao]

    clc
a = input('Enter Number of Cities need to travel: ');
b = 2;
t = xlsread('Data.xls');
if ~isempty(t)
    xlswrite('Data.xls',zeros(size(t))*nan);
end
city = randi(100,[a,b]);
xlswrite('Data.xls',city);

[Steve]

Your suggested codes working great enough, many thanks to you :).