The ambient temp sensor works by sending a variable resistance to the HVAC programmer.
18C/65F = 1380 ohms
24C/75F = 1050 ohms
29C/85F = 840 ohms
There is no way to calibrate the sensor. Since yours is reading higher than it should, it means that the resistance is lower than expected. If it was higher than expected such as to read 10* cooler you might just be able to clean the electrical contacts. But in your case, it seems like it is time to replace the sensor. They are less than $20 and are very simple to replace.
Don't feel bad... mine used to read 168* until I replaced it.