My wifi keeps indicating that it’s a weak security. How can I fix this?

Change the settings in your router to WPA2 AES or to WPA3 if your router supports that. Exactly how you change the router settings varies by the brand and model of your router. If you don't have instructions you can do an internet search for your router brand and model.

it's done in your wifi router, how differ from router to router, if it's very old it may not support better security

many has the a web interface where one configure them from a browser, if you google

<the brand/model of your router> configure encryption

then you will likely stumble across some guides