Merge / Insert a portion of another track

How do I merge into one track, a snippet of another track, and make it one permanent track?

For example, say I record a guitar track for a song, and call it “Guitar.wav”.
I’m not satisfied with the the beginning 10 or seconds or so of the recording, so i record just the beginning again, which I call “Begin.wav”.
Now I have 2 tracks.
I drag the “Begin.wav” up and over to the “Guitar.wav” track and place it where it should be in the timeline.
Now I want to save my change in a new (or existing) “Guitar.waw” file/track.

How do I do that?

Thanks in advance,