Dungeon Master’s Guide pg. 288 has this:
Adding New Abilities
A creator can add new magical abilities to a magic item with no restrictions. The cost to do this is the same as if the item was not magical. Thus, a +1 longsword can be made into a +2 vorpal longsword, with the cost to create it being equal to that of a +2 vorpal sword minus the cost of a +1 sword.
This rule also appears in the System Reference Document, e.g. here. The SRD is a collection of most of the core rules of the game, for easy reference. These materials have been released under the Open Game License, so there are numerous websites that host it (with a variety of value-added features like hyperlinking between sections and search functionality), but they are the official rules, just the same as in Player’s Handbook, Dungeon Master’s Guide, Monster Manual, Expanded Psionics Handbook, Deities & Demigods, and Epic Level Handbook. The d20SRD.org website I linked is my preference for the SRD (clean enough layout, doesn’t get muddled with homebrew as tends to happen on the wikis), and also includes open content from Unearthed Arcana.