This is known as a "re-spec", and it's reasonable on occasion.
While changing one's character levels isn't a standard rule in D&D 5th edition, it's not unreasonable to allow a character to change a poorly-built character on a one-off basis like this. In fact, the Adventurer's Guild organized play rules explicitly allow low-level characters to do this:
We recognize that many players start out with a pregenerated character, or might try out a character class, race, or other option, and then decide later on that it wasn’t the play experience they were looking for. As such, characters in the first tier (levels 1–4) can be rebuilt after any episode or adventure.
This is commonly known as a "re-spec" or "rebuild", and I've seen it used well in the first D&D campaign I ever played in. Most of our gaming group was new to D&D at this time, and many players made mechanically very sub-optimal choices as a result. When we reached level 10, the DM allowed us to change our characters once only, to ensure that nobody would be laboured with an ineffective character due to poor decision-making early on.
It's not unbalanced, since a re-spec is fundamentally no more powerful than a normal character of their level. It's mechanically identical to allowing the player to select a new character of the same level (something many DMs, myself included, allow when a character dies or retires), except that this way they retain their existing character identity and story, which is usually a good thing. And while AL currently only allows it up to level 4 (as noted by nick012000), you aren't bound to that rule unless you're playing in Adventurer's League.
However, I recommend it be allowed very sparingly, such as once per campaign. Allowing frequent re-specs would give characters too much flexibility (e.g. completely changing their character to suit each adventure or scenario), which would lead to an effective increase in power. It may also undermine the continuity of the game, and lead to much time being spent in character generation instead of playing the actual game.
You may like to offer the other players the chance to re-spec at the same time, just to be fair. However, again, it must be made clear that this is a one-off grace which players will not expect to be repeated. As DM, you are within your authority to bend the rules to accomodate players, but don't let them talk you into doing this often, or the rules lose meaning.
Youtuber and excellent Dungeon Master Matt Colville, of Running the Game fame, has in the past stated that he allows his players a great-deal of leeway in re-spec. He considers it important that characters get to play the sort of character they want to play, and that if the player's character concept doesn't match what they had mind when they made their decisions, that they should be able to change it. In one case he recently allowed a character to change a spell mid-session when it was discovered to be less effective than the player had assumed when they made that choice.