How do beta-blockers increase cardiac output?

They don't: Cardiac output is the product of heart rate and stroke volume. Beta blockers decrease heart rate and the strength of contraction (contractility). Thus, beta blockers would not be expected to increase cardiac output.

It does not: Beta-blockers do not increase, but rather decrease cardiac output. Their mechanism of action involves blocking the effects of Epinephrine and norepinephrine on beta-receptors within the heart and peripheral vasculature. By blocking these two hormones, a beta-blocker inhibits their effects including increased heart rate and contractility, the two factors that compose cardiac output.