the first step in LSMW tells you which program is actually used, I guess it is RCSBI020
You can go directly from there into the program, or you can use SE38 to get to this program.
There display the documentation and read the notes carefully.
for your convenience:
The input data must contain all items that are to be in the target system on the key date.
The report compares the items in the input file with the items in the taget system BOM on the key date, which you include in the BOM header of the input file. All items that are not included in the input file are deleted.