If a spell or ability has a variable number of targets, you can’t change the number of targets.

If the spell or ability has multiple instances of the word “target,” you choose which target you’re changing to Mizzium Meddler as Mizium Meddler’s ability resolves.

If Mizzium Meddler’s triggered ability targets a spell or ability with no targets, nothing happens.

The target of the spell or ability won’t change unless Mizzium Meddler fulfills all the targeting criteria. If a spell or ability has multiple targets, such as Send to Sleep, you can change only one of the targets to Mizzium Meddler.

If changing one target of a spell or ability to Mizzium Meddler would make other targets of that spell or ability illegal, the targets remain unchanged.

Mizzium Meddler’s triggered ability can target a spell or ability even if Mizzium Meddler wouldn’t be a legal target for that spell or ability. However, the target of that spell or ability will remain unchanged.

If Mizzium Meddler leaves the battlefield before its ability resolves, the targets remain unchanged.