The 5e Awaken spell allows a gain of intelligence and ability:
After spending the casting time tracing magical pathways within a precious gemstone, you touch a Huge or smaller beast or plant. The target must have either no Intelligence score or an Intelligence of 3 or less. The target gains an Intelligence of 10.
How does one get this Awaken spell to work on plants or animals with more than three (3) intelligence score?
The baboon and some giant creatures have four (or more) intelligence. Even the Shambling Mound would be an amazing pet (for a month) were it not so deeply clever. Yet, if high school taught us anything, baseline and expected intelligence is NOT guaranteed. Perhaps there is a work-around?
Give the target a Feeblemind spell first.
Enroll them in high-contact American sports.
Polymorph them into a more-stupid beast, then they change back smarter! Though StackExchange gave this one a complicated answer.
Admit that the author is probably making a bigger deal out of this than necessary and just House Rule the damn thing.
Repeat of Question for Exchangers of Stack: how does one imbue bonus intelligence (and myriad abilities) to plants &/or beasts with a wee bit ‘too much’ intelligence?