It agrees with the noun it's modifying, although not in a way which is currently taught in this course.
"Iliyopita" and "uliopita" are both relative constructions that break down as follows: subject prefix, past tense -li-, relative infix, verb stem -pita. Literally you're saying "the week that passed."
"Wiki" is N- class (like most English loan words), so its singular subject prefix is i- and its relative infix is -yo-: "Wiki iliyopita." Compare this to "mwezi," which is MI- class and so has the singular subject prefix of u- and the relative infix of -o-: "Mwezi uliopita" (last month)
Thankfully with practice, these patterns do start to come naturally. :)