Shorten is a serviceable word for describing how to reduce the extent or length of something, but some synonyms are available to use in its place.
Abbreviate stems ultimately from the Latin verb abbreviare, the root of which is from brevis, meaning “short”—the same word from which brevity (“briefness”) and brief are derived.
Abridge, which has nothing to do with bridges (it has the same origin as abbreviate), is often used in the sense of diminishing effect or strength or shortening a written compensation by excising parts.
To curtail is to limit or reduce as if by cutting (its obsolete predecessor, curtal, referred to cutting an animal’s tail short); its derivation is curtus, Latin for “short”—which came to be used in English as curt, an adjective usually applied to a brusque statement. Truncate is ultimately from the Latin word truncus, the source of trunk (as well as truncheon—the original term for a billy club—and the rare word obtruncate, which means “cut the top from”). The original sense is an adjective meaning “with square or even leaves”—leaves that appear to have been artificially shortened and straightened.
Elide means “omit”; it usually pertains to removing a letter, word, or phrase from a document but also has a general sense of “shorten”; the noun form is elision. Syncopate means “cut short,” but it also applies to the linguistic process of syncope, in which part of a word is elided, as in g’day for “good day.” It’s also the verb form of syncopation, which refers to music rhythm based on giving stress to weak rather than strong beats.
There is also a group of short words—often, in their pronunciation, suggestive of abrupt action—that refer to cutting something short, including bob, chop, clip, crop, cut, dock, lop, and snip. Other terms referring to cutting, often in reference to vegetation, include mow, pare, prune, and trim; shave and shear are similar.
Précis, a noun referring to shortening or condensing (from French, and the ancestor of precise), is also a verb; other terms are compress and contract, as well as condense and its close synonym digest (from the sense of the word, also associated with assimilating food into the body, of arranging and dividing). To abstract, profile, and summarize are similar actions, though they involve outlining content rather than reducing its length. Likewise, to epitomize is to serve as an example rather than to actually reduce. Shrinking, meanwhile, involves literally reducing in size rather than abbreviating.