I have two phone lines. I want to disconnect 2nd line but need to rewire that jack to main number. The jack has green red top l to rt blk yello bottom left to right. I tried switching the yellow to blk and green to red. But it didn't work. What am I doing wrong? How should they go?

Red and green is usually line 1, yellow and black is usually line 2.
You may have to go to other jacks and the box outside to find wires disconnected, if swapping pairs doesn't work.
Installers don't always have both lines on both pairs to each jack, keeps someone from tying in on that line, or just lazy.

I thought it was yellow to red and black to green.

AbleComm: Phone wiring color codes (http://www.ablecomm.info/colorcodes.htm)

Red and Yellow are the Ring Terminals
Black and Green are the two tip terminals

Red = R1, Green is T1
yellow is R2 and Black = T2

1 and 2 are for line 1 and line 2.
T = Tip and R = ring, designations from the 1/4" phone plug days of the operator

To switch line swap (Red and yellow) and swap (Green and Black)

Just like labman says.