Techniques and ingredients to make marinara sauce from canned spagetti sauce

If for some reason, you only have canned goods available in your area, I would start with canned tomatoes -- either crushed, diced or a mix, depending on how chunky you like your sauce. (some people would argue that whole peeled have better flavor, but you end up with a can with a lot of juice in it so requiring more cooking (or reserving the juice for minestrone or some other purpose)

If canned spaghetti sauce is all that you have, or you're trying to use up a case, you don't necessarily need to cook it down for a long time. You can instead pull the pasta early, and finish cooking it in the sauce; the pasta will then soak up some of the moisture thickening the sauce. You may want to reserve some of the pasta water in case it ends up a little too thick by the time the pasta is cooked through.

As for spicing, yes you can modify things if it's bland, but if it's an off flavor ('tinny') or something too strong, it's more difficult. Sometimes a little bit of sugar can help if it's a touch sour, but you can also cook down some onions or finely diced carrots and mix that in to get it some extra sweetness (and texture, and other flavors)

And if the issue is price differences -- If you have a Wegman's near you, try their store brand sauces. It's like $1/jar (about the same as what a can of sauce costs), and it's quite good.

You can not "spice up" any bottled sauce to make it a marinara, because a marinara is inherently a very simple and basic tomato sauce. It is tomatoes, garlic, olive oil, basil and a little salt,pepper, sugar. Even if you could take all of the extra crap that is in Prego out it would still be Prego, (any canned or bottled sauce will never equal a freshly homemade marinara sauce).

Get a can of quality whole tomatoes (no extra ingredients, I like Cento brand) empty them into a bowl and then crush them to a fine pulp with your hands. In sauce pan heat some olive oil and ad some (3, 4 or 12 cloves?) crushed or finely diced garlic, gently cook on medium heat until just softened, careful not to get it to hot that you burn the garlic. Add the tomato's and their juice, a pinch of sugar (I like brown sugar), a big sprig of basil, salt and pepper to taste, bring to a gentle simmer and reduce until thickened for a least 40 minutes but longer is better.

Take out the basil sprig, it has done its job. Adjust the salt and pepper if needed and freshen it up with a little olive oil. That's it, next time you can make adjustments for your wife's tastes, like some red pepper flakes or a little tomato paste or oregano etc. You will never find a pre-made sauce to equal your homemade sauce.