You can do it in two ways.
1. One you can declare some Public properties on user control which you can access on ASPX page.
2. You can add one event in UC which you can pass/bubble up to aspx page with that you can pass data.
See below inside user control
public delegate void mydelegate(string str);
public event mydelegate btnHandler;
protected void UC1btn1_Click(object sender, EventArgs e)
{
if (btnHandler != null)
btnHandler("");
}
In aspx page
UC11.btnHandler += new UC1.mydelegate(UC1_Click);
public void UC1.mydelegate UC1_Click()
{
}