If you only need an accented character every now and then, there is no need to change your full keyboard layout. Inserting Accented Characters with an English Keyboard Layout.
There are several ways to configure your keyboard to type in the Spanish accented letters and upside-down punctuation (á, é, í, ó, ú, ü, ñ, ¿, ¡) and which one you use depends on the frequency with which you need these letters. How to Type Spanish Letters and Accents (á, é, í, ó, ú, ü, ñ, ¿, ¡)
