How do I delete an old iPhone on icloud

Log into https://appleid.apple.com/account/manage

Then in the Devices section click on your device.

Also go to the same place to remove a device from your account.

Or go to Settings > [username] > [device name] on an iPhone or iPad

Also see: Remove a device from Find Devices on iCloud.com - Apple Support

I lost my se phone bout one mouth ago. Bought a new se3 and I need to know how to get my contacts music and things transferred to this phone

To remove a device from your account, Go to Settings then Tap (Your name) then tap the device you want to remove. Now do whatever you need to there.