You will struggle to repair the tyre if the nailis at the edge, there are repairable limits on what can be done.
If you have had a slow leak previously check the rims, they could be corroded and letting air pass at the bead of the tyre (this is what is happeing on mine just now). It is easy to test – remove wheel from car, inflate tyre then spray some soapy water round the wheel and look for bubbles.
conclusion – Replace tyre you probabley wont be get it repaired if it has a nail in or near the edge but check out you rim first at it may need some work so tha the new tyre seals propery.