I have a 20 amp outlet (3 holes with one at 90 degrees) and want to convert it to a standard 15 amp outlet. Can that be done and how do I do that.

If this is in U.S. and there are 3 slots and its fairly big then what your looking at is 220v. What type or outlet are you looking to change it to ?

Shut off power, remove 20 amp and install your 15 amp receptacle on this 20 amp circuit. Its OK.

Does it look like the plug on this page?

Ref:

VWvortex Forums: Welder Plug adaptor question (http://forums.vwvortex.com/zerothread?cmd=print&id=3968412)

If this is standard USA home wiring there should be no need to switch this out. The 20 a receptacle can take a 15amp plug. Tell us more details, maybe we are missing the mark until we hear more. If this is what you have, Google Image Result for http://cache1.smarthome.com/images/4537w.jpg (http://www.google.com/imgres?imgurl=http://cache1.smarthome.com/images/4537w.jpg&imgrefurl=http://www.smarthome.com/4537W/Acenti-20-Amp-Duplex-Receptacle-Alabaster-AC220-W/p.aspx&h=275&w=275&sz=6&tbnid=Kj796JBRevY8cM:&tbnh=114&tbnw=114&prev=/images%3Fq%3Dpicture%2Bof%2Ba%2B20%2Bamp%2Brecepta cle&hl=en&usg=__IOZd2kdDOBwdwiDRo1Je9KJ89jA=&ei=jemJS7mBApKINKaY8aUB&sa=X&oi=image_result&resnum=3&ct=image&ved=0CBYQ9QEwAg) you don't kave to do a thing

If this is in U.S. and there are 3 slots and its fairly big then what your looking at is 220v. What does "big" have to do with "220", or more accurately 240v?
The OP specifically stated 20A. That is no bigger than a standard duplex receptacle.
240v has nothing to do with physical size.

KL, this is what you need to know:
If this is standard USA home wiring there should be no need to switch this out. The 20 a receptacle can take a 15amp plug. Tell us more details, maybe we are missing the mark until we hear more. If this is what you have, Google Image Result for http://cache1.smarthome.com/images/4537w.jpg (http://www.google.com/imgres?imgurl=http://cache1.smarthome.com/images/4537w.jpg&imgrefurl=http://www.smarthome.com/4537W/Acenti-20-Amp-Duplex-Receptacle-Alabaster-AC220-W/p.aspx&h=275&w=275&sz=6&tbnid=Kj796JBRevY8cM:&tbnh=114&tbnw=114&prev=/images%3Fq%3Dpicture%2Bof%2Ba%2B20%2Bamp%2Brecepta cle&hl=en&usg=__IOZd2kdDOBwdwiDRo1Je9KJ89jA=&ei=jemJS7mBApKINKaY8aUB&sa=X&oi=image_result&resnum=3&ct=image&ved=0CBYQ9QEwAg) you don't kave to do a thing


If what you have is not the one in the pic PLEASE post a pic, drawing or link to the one you have. If the receptacle h as writing on it you can post that as well.
Until then this is all just speculation.

What does "big" have to do with "220", or more accurately 240v?
The OP specifically stated 20A. That is no bigger than a standard duplex receptacle.
240v has nothing to do with physical size.

From the Op's description it sounded like a dryer plug. ( recepticle). Rated at 20a. If that is the case then we need to know what exactly they are looking to do. Install a reguler 3 prong 120v 15a outlet or change the circuit or what? Some with no experience seem to think a plug is a plug. And we know that's not true and also don't want to give advice and end up someone gets hurt. A 220 plug like dryer plug is usually larger then a standard 120v.