Hello, I have a second phone line for my computer, I am dropping the second line and getting DSL. How do I rewire that second line to my main line? Thank you.

You should not have to do any rewiring at all.

The "main line" typically has enough wires in it to handle 2-8 lines... so your line 2 is using two of them that just won't be used anymore. Your line 1 will remain connected as it is.

The second line would have been connected to the yellow and black wires. You want to rewire it to the red and green. Unfortunately, I can't say which is which. One is ring and one is tip. Somebody explained here a long time ago. Maybe use the search, phone + ring + tip.