Convert advance tax to self assessment tax

It is pay time mistake. I have to pay (300)SELF ASSESSMENT TAX. By mistake I have selected (100)ADVANCE TAX during the tax payment.

No need to convert this. Since you have paid the tax on 31.03.2014 so it would be considered as advance tax. Self assessment tax is paid after the date of 31st march.

Further if you was liable to pay advance tax then interest will be charged accordingly section 234A, B, C. 

There is not matter that you have shown as advance tax and if you shown this as self assessment tax then interest would not be charged. 

If you was not liable to pay advance tax then and you have paid then no matter. No any interest will be charged. You will show this in ITR and rest amount of tax will be paid as self assessment tax.