Como cambiar una palabra por otra

[relaxamos]

Para tontos:

static String replace(String str, String pattern, String replace) {
int s = 0;
int e = 0;
StringBuffer result = new StringBuffer();

while ((e = str.indexOf(pattern, s)) >= 0) {
result.append(str.substring(s, e));
result.append(replace);
s = e+pattern.length();
}
result.append(str.substring(s));
return result.toString();
}

Para listos:
String s = "citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.49.9988";
s = s.replaceAll("\\{summary\\}", "download");