Hi@Fannyfanny,

Thank you for contacting our community.

According to the translation of Google Translate, I think that your question is that you want to change Wi-Fi password of RE455. If there is any discrepancy, please describe it in English.

Once the Range extender is connected to your router’s Wi-Fi, it will use the same wireless password. 
However, some models have been updated to allow customization of their own wireless password and encryption type. 

You can check the supported models and get detailed instructions on how to set a different password for the extender at this link: Support Customizing Password and Encryption Type on Range Extenders.

But if you cannot log into the extender in the Tether app, follow these steps to troubleshoot:
    1) Visit tplinkrepeater.net and log in with the same login password to ensure it is correct. 
    2) Update the Range Extender's firmware through its web management interface, go to Advanced > Advanced > System Tools > Firmware Upgrade to check for updates.
    3) Update your Tether APP to the latest version.
    4) Reboot the extender and restart the Tether app. 
    5) If the extender uses a Static IP, ensure its IP is in the same subnet as your router.
    6. Reset the extender and reconfigure it following the instructions on the Tether app. 
 

Thank you.