1. First create a custom Geographic Transformation as following:
You can select the input GCS and output GCS from ESRI projection list:
GCS => World => WGS 1984
PCS => National Grids => Malaysia and Singapore => SVY21_Singapore_TM
2. Project Data: select From_WGS84_to_SVY21 for the last option(just now created)
3. It’s a reverse step to do the SVY21 to WGS 1984 conversion.