Use this guide to remove the main board, which is probably the most delicate part of the operation. The SD card board is on the right side under the main board, and should be accessible once you remove the former. There are 3 connectors on the SD board: One flat to the main board, one wired two pin connector to the power board (top right) and one small flat that goes somewhere on the chassis (bottom left). Once all connectors have been detached, you can undo the 4 screws that hold the SD board, take it out, and fit the replacement.
Take utmost care not to damage the flat cables and the connectors. Some flat connectors have locking tabs that must be flipped up before you can extract the cable. Some large flats such as the one that connects to the sensor board have press fit connectors that pop out vertically (at right angle to the board), these should be carefully lifted out with the tip of a plastic spudger. Putting them back is easy, just line it up correctly and gently push it in place with your fingertips until it "clicks".
Once the replacement board is fitted, just follow the guide in reverse to put everything back together again.