First get permission from the supplier to change connections.
Then in a clean dry place, open the transformer and change the internal connections. On the back of the center low voltage bushing you will find two cables or straps. One must be moved to one line bushing and the other must be moved to the other line bushing. Check your X1, X2, X3, X4 connections in your text book for the proper connections. Use an Ohm-meter to check continuity before making the new connections. (I saw a crew mess that up on a bank of 100 KVA pigs and stall the generator in the small diesel generating plant that supplied the area.)
If this is oil filled, all tools must be clean and dry. The workers should wear oil proof gloves, both for their own protection and to avoid contaminating the transformer oil. A tiny amount of dirt may lead to early failure of the transformer, possibly in hours or weeks.
If the transformer is oil filled with the HV bushings in the top, be careful not to damage the high voltage leads when you remove the top. There should be enough slack to move the top enough to change the secondary connections but not much more.
After the change you will have 120 V transformers with the center secondary bushing not used.
If this is your first time, try to find someone to assist you.
The supplier may prefer to supply transformers that are already configured for 120V rather than allowing you to make the changes in the field. There may be warranty issues if you raise the top of the transformer.
Utility workers routinely do this reconnection in the field but utilities have much better margins than contractors.
Bill
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"Why not the best?"
Jimmy Carter