Set the 'BackUp Expires' to be after "15" days, that sure that it is the left radio button that is clicked, like in the attached screenshot. LiteSpeed will not overwrite this file for this number of days.
If are using a LiteSpeed script directly (instead of the LiteSpeed Console UI) to run the backup, then you use the argument of --RetainDays
So you would add to your script the argument of --RetainDays=15
If you are using an extended stored procedure, it is done with an argument: @retaindays
That argument, like the other arguments, is not case sensitive.