Connecting to specific Wi-Fi channels?
In a network environment with multiple Wi-Fi access points with same SSID, iOS/macOS device automatically connect to the one with the best signal strength. This should happen automatically without any intervention from user side.

If you find the connection to be poor, go to Settings -> Wi-Fi (on iOS)/System Preferences -> Network (on macOS) and toggle Wi-Fi off and on again. This should resolve your issue.