You can’t directly convert since they measure different things. The most useful explanation I’ve found is that lumens measure light output at the source, while candelas measure the light emitted per unit of solid angle, a quantity that conveniently does not vary with the distance from the source of the light. So, they measure different things, and there’s no direct conversion. Update: I didn’t find that answer very satisfying either, so I worked out the conversion details and made a conversion calculator for you to use.

Light bulbs and LEDs sold for illumination tend to carry ratings in lumens. Indicator LEDs tend to be rated in candelas.

You can also use this table to get an approximate conversion from candelas to lumens. Find your LED beam width in degrees, and divide the candelas number in your specs by the cd/lm factor listed for that beam angle to get lumens.

beam angle	cd/lm
5		167.22
10		41.82
15		18.60
20		10.48
25		6.71
30		4.67
35		3.44
40		2.64
45		2.09

Here’s an example of how to use the table:
Page 89 of the widgetco catalog offers two white LEDs. The specs are listed as follows:
Ultra-bright white LED: 16,000mcd 5° beam angle
Mega-bright white LED: 9,000mcd 10° beam angle
You want to purchase the one that will give the most light flux in your solar accent light. Which one do you select?
Solution:
Ultra-bright gives 16cd / 167 cd/lm = 0.10 lm
Mega-bright gives 9cd / 42 cd/lm = 0.22 lm
Wow, Mega-bright throws over twice the flux as Ultra-bright. Would you have guessed from the catalog listing?
The caveat is that you have no idea from the specs listed what angle the maker used to measure the intensity (millicandelas). Theis exercise assumed that it was measured over the entire beam angle. That’s reasonable–you would expect the manufacturer to publish the highest possible intensity spec. But there’s a couple sources of error in this approximation: first, there’s light outside the beam half-power angle, and second, the light inside the beam half-power angle isn’t guaranteed uniform by any means. So we’ll call this an approximate conversion. Your mileage may vary.