If you wish to extend your education loan from the bank, simply write to the bank manager using compelling language to describe your circumstances.
Start by addressing the letter in a formal manner:
If you know the name of the recipient, use that. If not, you can always use "To whoever it may concern,"
In the body of your letter, summarize the original terms on the loan; for example how much money you borrowed, what interest rate you were paying, and what the repayment schedule you agreed to was.
In the next paragraph, explain the circumstances which have lead you to request an extension: Why will you have difficulty making payments based on the original schedule, how much time do you anticipate you need? What will you be willing to offer in return for an extension (perhaps paying a higher interest rate or a penalty fee).
Even if you would like to avoid paying an additional charge, sometimes it is a good idea to simply inquire as to whether that will be the case.