How to decide winner in football if match ties in extra time?

If a match is tied at the end of normal time, Law 10 allows three methods to determine the outcome of the match:

When competition rules require a winning team after a drawn match or home-and-away tie, the only permitted procedures to determine the winning team are:

• away goals rule
• two equal periods of extra time not exceeding 15 minutes each
• kicks from the penalty mark

A combination of the above procedures may be used.

In your example, extra time has already been played so it would be settled either via the away goals rule or kicks from the penalty mark (commonly called a penalty shoot-out); which of these are chosen will depend on the competition.