How can a step down transformer be used for step up or vice-versa ?

A transformer usually has two windings denoted as HV(High Voltage) and LV(Low voltage) windings. The transformer is used to transfer the power fromn one voltage source to another. So the same transformer can be used as a Step Up / Step down transformer. For e.g , assume you have a transformer whose H.V winding is rated for 11 KV and L.V winding is rated for 6.6 KV. If you connect a 6.6 KV source to the L.V winding then you get a 11 KV output at the H.V winding - this is a step up application.

Similarly , if the source available was 11 KV , then by connecting it to the H.V winding you get a 6.6 KV output at the L.V winding - this is a step down application.

Hence, the same transformer can be used for step up / step down. It depends on the source/input voltage available at site.

You'd have to reverse the input/output leads to the transformer. Wire the transformer such that what was its input is now the output.

yes. tho one way is usually more effient than the other depending on the type of transformer. a torroid transformer is best.