If it is a Marlin Ballard, you typically can flip the firing pin from rim to center-fire. The firing pin should be a flat piece of steel, held in by a cross screw through the breech block assembly. You may be able to flip it without removing the block but probably will have to pull the block to do it. 
Sequence; (from memory, it's been many years)
1. Remove the lever screw
2. Pull the block out the bottom, fiddling with the extractor if needed to get the lever and block assembly out. 
3. Remove the upper cross screw and the pin should come out.
4. Flip the pin and put it back together. There is a trick to get the extractor back in. It must go in with the block and will probably take some wiggling to get "around the corner".

If it's a Ball & Williams, R. Ball, Merrimac, Brown or Dwight & Chapin instead of a Marlin you are stuck with rimfire unless you modify the block which I would ask, "please don't mess up an original gun, sell or trade it for something that will do what you want."

I'll caution you that the rifle is unlikely to be strong enough to be converted to .38-55.  I regretfully did one many years ago when I was too stupid to know better.  I sold it with the caution that it was a weak action, but I have seen it sold 3 times without that caution since then.  Fortunately it hasn't blown up that I heard of.  I tried to buy it back last time I saw it, but had already been sold to yet another owner.

Look up the difference between cast frame and forged frame Ballards.  The easiest way to tell is the area of the action underneath the barrel ring is hollowed out on a cast action, and filled in solid and smooth on a forged.

Quite simply, cast actions are only suitable for rimfire cartridges or low pressure pistol cartridges, and forged are suitable for centerfire rifle cartridges.

As Jerry said, if it isn't what you want, sell it and buy a suitable one instead of modifying an original into an unsafe configuration.

If you cock the hammer and look at the rear of the firing pin it will be rectangle shaped if it's reversible. And the screw holding the firing pin in place will be just above the top of the action. A CF block has the screw much higher above firing pin centerline.
Most cast actions will have a 3 line rollstamp on the left side, and forged actions mostly are 2 line rollstamps. But there are exceptions both ways, so this is true for maybe 99% of the actions.
Depending on breech block height you may be able to remove the firing pin screw without dropping the block if you can see the entire head of the screw. Otherwise the block needs to come out to switch to CF.