Legacy System Migration Workbench (LSMW) is a SAP tool which you can use to transfer data into an R/3 System.
This method is very widely used during cut-over, to transfer legacy system data in to SAP. You can use same functionality to do mass changes to existing data in SAP as well. Please click here to see how you can use MASS transaction in SAP.
Eg: If you want to change any field value in the material master, you can download those data from the table in to an Excel file. Then you can change relevant values in the Excel sheet. After that using a LSMW, you can write new values back to SAP.
Below document will explain you on how to create a basic LSMW. Based on those steps, you should be able to create your own LSMW.
If you want to find out, how you can transfer your LSMW from one client to the other, please click here.
Please feel free to post your suggestions, feedback and comments!