Rim Replacement
If your rim is dinged, replace it. A badly bent rim can sometimes be trued, but it will be weakened.
You need the same tools for as for a wheel build.
If your new rim has a different ERD from the old one, you need to follow the procedure for building a new wheel--you will have to buy new, different length spokes and start from scratch.
If your new rim has the same ERD as the old one, you can skip the lacing procedures and substitute these.
The procedure to replace a rim with the same ERD:
1. Remove the wheel from the bike, all the rubber from the wheel, and the cogset.
2. Clean everything you can reach.
3. Loosen the nipples on the old rim until the spoke is barely attached. At first, loosen each nipple half a turn, rotating around the wheel. When the spoke tension is relieved, you can loosen more than half a turn at a time.
4. Inspect the spokes and nipples, and replace any that are damaged. It is best to buy all new nipples. You can make the change later, when you move the spokes to the new rim.
5. Tape the 2 rims together with the valve holes directly together and the offset, if any, in the right direction.
6. One by one, disconnect the spokes on the old rim, treat the threads with a spoke prep (or beeswax), and reconnect the spoke to the same hole on the new rim.
8. Put it all back together. Adjust the brakes and rear derailleur if necessary.