How do I change a two prong outlet to fit a four prong dryer cord for a stackable washer/dryer unit? Will this cause a problem with the circuit? I have 100 voltage according to the breaker box.

That four prong dryer is almost certainly 240 vac and your outlet is not 240. Do you have 240 anywhere in the home and where do you live?

You really need to help us here.

Are you talking about a standard 120 V receptacle?

If so, you would need to start at the beginning, in the main panelboard. You would need two slots, one directly below the other.

A 240 Volt (two pole) breaker rated for the needed amperage (the amperage you need will be on either the instructions and the electrical faceplate on the unit itself.

Then you would need a four wire cable, listed for the amperage needed and a NEMA receptacle that matches the plug configuration.

Because of the size of the receptacle, you will also need to install a 4 inch device box and a new faceplate.

Question, is this work you know how to do?

you would need to start at the beginning

I agree but the beginning would be reading the electrical tag.

This sounds like a 120V apartment size stackable washer/dryer.

" i have 100 volatge according to the breaker box" that might be an apartment in Tokyo