It depends on what you want to change. To change the meaning of a kanji, use the three dots to bring up the menu and choose customize meaning (currently, the dots are not visible in the dictionary, only in quiz results and in lists). To correct anything else (readings, stroke count, stroke order, radical, or any of the various index numbers), send a DM to @マイコー or post in the forums. If you encounter the issue during a quiz, you can use the in-quiz issue reporting system: tap (?) and fill out the form. Here’s how I typically do it:
Be aware that the kanji dictionary is sourced from a third party. Corrections should cite a reliable reference. Michael prefers Jitenon.
The above procedures apply only to kanji. The process is similar but different for words, sentences and grammar rules. Ask if you can’t figure it out.
P.S. if the problem is with a mnemonic rather than any of the above, there is no way to suggest edits, because mnemonics are user supplied content. If both you and the author are on Discord, you may be able to send a private message. Otherwise, all you can do is hide the problem mnemonic by tapping the eye icon next to the author’s name.