Here are the six mistakes in the letter and their corrections:

1. **"Ishouldn't"** should be **"I should"**.
- Correction: "I should give you some advice, so"

2. **"a bit of confusing"** should be **"a bit confusing"**.
- Correction: "it can get a bit confusing for visitors."

3. **"you has"** should be **"you have"**.
- Correction: "Firstly, you have to take off your shoes when you go inside."

4. **"you didn't have to bring any"** should be **"you don't have to bring any"**.
- Correction: "but you don't have to bring any, we have extra pairs for guests."

5. **"you have use"** should be **"you have to use"**.
- Correction: "Then you have to use different slippers in the bathroom and in the garden,"

6. **"You should worry"** should be **"You shouldn't worry"**.
- Correction: "You shouldn't worry-I'll be there to help you."

Here is the corrected letter:

Hello Mi,

I'm so excited about your trip. It's going to be amazing. I should give you some advice, so you can prepare for Japan. We have lots of customs and it can get a bit confusing for visitors. Firstly, you have to take off your shoes when you go inside. You shouldn't wear slippers in the house- but you don't have to bring any, we have extra pairs for guests. Then you have to use different slippers in the bathroom and in the garden, but you'll get used to it! You shouldn't worry-I'll be there to help you.

All for now, gotta go....