Use the Roku mobile app on your phone
Connect your phone to the same WiFi network the Roku was previously using
Open the Roku app
Tap Devices
Select your Roku device
Use the app as a remote
Go to Settings
Select Network
Select Set up connection
Choose Wireless
Select the new WiFi network
Enter the new WiFi password
Wait for Roku to connect
If the Roku is not on the old WiFi, create a mobile hotspot with the old network name and password
Connect the Roku to that hotspot
Use the Roku app to change the WiFi settings to the new network
If needed, use an Ethernet connection on Roku models that support it
If the Roku has a physical reset button, reset it and set it up again with the new WiFi
If your TV supports HDMI-CEC, use the TV remote to navigate Roku menus
If none of these work, use a universal remote compatible with Roku