var temp: integer;

..................

for i:=1 to n do
for j:=1 to n do
begin
if a[i,i]<a[i+1,i+1] then
begin
temp:=a[i,i];
a[i,i]:=a[i+1,i+1];
a[i+1,i+1]:=temp;
end;
end;