The word I am thinking of does not seem to fit (or be appropriate for) the situation. Would you please tell me a better word to use?

The way you have already said/stated it is perfectly fine. There is no need to change it.

Maybe say 'I need to replace *the word* with a more appropriate word'

“The word I used wasn’t really correct, so I need to change it.” It depends on why you want to avoid using the word appropriate. Saying “it wasn’t really appropriate” can mean two things: 1) it was rude/disrespectful/offensive or socially unacceptable (for example a sexual comment) 2) it was awkward or not quite correct, such as a non-native speaker using a word that is slightly wrong but still understandable, or a native speaker who can’t think of the exact word they want to use and uses a word with a similar meaning. Either way, “it wasn’t really appropriate” is a polite and respectful phrase. There would never be a problem with saying it in most conversations and settings. It’s hard to suggest an alternative because that’s the phrase native speakers would usually use. If there a specific reason why you don’t want to use it, that would be helpful to know so we can offer a suggestion.

I donot wont to use appropraite ....