Usually tourist visas in cant be extended in Japan.
You would need to leave Japan and come back with another one (or get another one on arrival, depending on which country you come from).

Other visas can usually be extended if you have a valid reason to stay in Japan (usually a continuation of the purpose of the original visa i.e. work, study etc )

You would need full documentation to support the extension - usually similar to what was required to get the original.

Usually you can apply for a visa extension up to 60 days before the expiration of the visa.

Applications need to be made in person at a Japanese Immigration Center.