Open the script editor and modify the Vocab module (at the top). The %s 's are placeholders. They get replaced with certain in-game variables, so it's best if you keep those.
Modifying the vocab module is pretty simple and straightforward. Just change the text in the quotes (" ") and it'll change it in game. However, there are some that have %s 's (or %x$s 's) that change inside the game. Here's a guide to what those tags represent:
CODE
ExpNext = "To Next %s" # Tag will change to the vocab for Level (database)
PartyName = "%s's Party" # Tag will change to party leader's name
Emerge = "%s's party was attacked!" # Tag will change to monsters' names
EscapeStart = "%s attempts to escape..." # Tag will change to your party's name
Victory = "%s was victorious!" # Tag will change to PartyName (above)
Defeat = "%s was defeated." # Tag will change to PartyName (above)
ObtainExp = "%s EXP were received!" # Tag will change to amount of Exp obtained
ObtainGold = "%s%s were obtained!" # Tag1 will change to amount of Gold obtained, Tag2 will change to the vocab for Gold (database)
ObtainItem = "%s was found!" # Tag will change to item found
LevelUp = "%s is now %s %s!" # Tag1 will change to actor's name, Tag2 will change to the Vocab for level (database), Tag3 will change to level achieved.
ObtainSkill = "%s was learned!" #Tag will change to skill learned
DoAttack = "%s attacks!" # Tag will change to battler who attacked
DoGuard = "%s is guarding." # Tag will change to battler who guarded
DoEscape = "%s ran away." # Tag will change to battler who ran away
DoWait = "%s is waiting." # Tag will change to battler who waited
UseItem = "%s uses %s!" # Tag1 will change to battler name, Tag2 will change to the item name
ActorDamage = "%s took %s damage!" # Tag1 will change to affected actor's name. Tag2 will change to the damage taken
ActorLoss = "%1$s lost %3$s %2$s!" # Tag(%1$s) will change to actor's name,Tag(%3$s) will change to mp damage taken, Tag(%2$s) will change to Vocab for MP (database).
ActorDrain = "%1$s drained %3$s %2$s!" # Tag(%1$s) will change to actor's name, Tag(%3$s) will change to damage drained, Tag(%2$s) will change to Vocab for HP (database)
ActorNoDamage = "%s took no damage!" # Tag will change to actor's name
ActorNoHit = "Miss! %s took no damage!" # Tag will change to actor's name
ActorEvasion = "%s evaded the attack!" # Tag will change to actor's name
ActorRecovery = "%1$s recovered %3$s %2$s!" # Tag(%1$s) will change to actor's name, Tag(%3$s) will change to recovery rate, Tag(%2$s) will change to vocab for HP
# Tags for EnemyDamage to EnemyRecovery have the same format as ActorDamage to ActorRecovery
ActionFailure = "There was no effect on %s!" # Tag will change to battler's name
If there is more than one tag with the same indicator (e.g. %s attacked %s for %s %s) in a line, you can change the order by changing it from %s to %x$s , where x is the numeric value of the tag. For example, if you changed ActorDamage from "%s took %s damage!" (comes out "[Actor name] took [number] damage!") to "%2$s took %1$s damage!", it would come back as "[number] took [Actor name] damage!" Changing the number in tags that already have numbers changes their order.
Please correct me if you see I made any mistakes above.
As for changing the message delay, that's slightly more complicated. It's not that bad, but there are a lot of areas where a message appears in Scene_Battle, so it really depends on which messages you want to delay.