There are various different ways.
One simple way without using VBA is to add a "helper" column. Let's say we want to keep rows 1,4,7,... and delete rows 2,3,5,6,8,9,...
We can make use of the MOD function (which returns the remainder of a number when divided by another number (3 in this case) and the ROW() function, which returns the current row number.
So, if we use a "helper" column, and put this formula in and copy down for all rows:
Code:
=IF(MOD(ROW(),3)<>1,"DELETE","")
then all rows we want to remove will have the word "DELETE" in them.
Then you can either use Filters to filter out the "DELETE" rows, or simply sort the range so that all the "DELETE" rows are together and can easily be deleted.
Another alternative would be to use VBA code to loop through your range and delete unwanted rows, using the same sort of logic.