There are many ways to transform between coordinate systems. One of the simple ways is using specific tool for transformation such as TRANSKOORD from BAKOSURTANAL (Indonesian Survey and Mapping Coordinating Agency). This tool is relatively simple because we do not need to install, just click on the EXE file. To download the tool, just click on the download link at the bottom of this post. The interface is in Bahasa Indonesia however.

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Run the tool by double-click on TransKoord.exe

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The first metode is INTERAKTIF. Choose Geografis ke UTM

Fill the name of the point (Nama Titik), Latitude (Lintang), and Easting (Bujur), Select Datum (for example WGS84)