I’m not familiar with LSMW but this generally how we deal with multiple items in BDC.
I assume you went to SM35 and made a recording of the BDC you want to run and created a program using an input file. I’m guessing your issue is you have multiple line items and you want to modify some or all the lines and can’t determine which line on the screen relates to your input data.
Typically we read the file the screen is displaying using a select with appropriate where conditions into an internal table, then sort the table into the same sequence it appears on the screen. Then determine which table entry is the one you want to modify, sy-tabix will contain the index into the table of your entry; use it to put an X in the appropriate SELX field to select your entry.