How do i switch screens from lenovo to coby?

Lenovo is a brand of computer. Coby is a brand of HDTV or monitor. That is all that can be replied from your information besides guessing:
You have some computer, maybe even a laptop called Lenovo, some unknown model. There's some unknown operating system running it. In that system are settings for video output. Some people use a second screen and can set up that by going to video properties or resolution or settings menu.
You have some video monitor called Coby, some unknown model. There might be a connection cord between it and the computer or laptop.

Replace monitors? Just swap them. The OS will detect the change and adjust accordingly. You may have to tweak resolution if needed.

Dual-monitor setup? Right-click on the desktop. There's a menu option there about graphics or screen and within that, an option to switch screens.