So last name is Lenart. But was told it was shortened when grandparents came to USA. How do I find or where do I go to find. So not sure if it was Lenartowski, or another. All I have is first names and date of birth and date of death, in Ohio, USA.
Start with what you know and work your way back. Have you found your grandparents in all of the relevant censuses? Do you know when they immigrated, and via which port? Were they naturalized? If yes, do your parents or aunts/uncles have any of the paperwork relating to that?
The good news is that such a name change, if it occurred, likely happened on this side of the pond, meaning that you don't need to deal with documents or websites in a language you don't know. But do keep in mind that the ubiquitous myth about "they changed my ancestor's name at Ellis Island" is completely fictional. Nobody -- at Ellis Island or elsewhere -- had the authority to change anyone's name. Only the person himself could make that decision, and most people only did so after living with their old names in their new contexts for a while.
Because of the availability of Ellis Island and other US immigration records you might find your task to be easier than you think. Try entering variants of the surname into the search field and (in FamilySearch) filter on Immigration records. The use of wildcards (e.g. using Lenart* or Ln*r instead of the precise name) has often produced results for me.
Things are much more difficult when it comes to immigrants who came to England, say around the early 1900s. There were no immigration records at all, birthplaces (in the census records) are usually vague and anglicised names (e.g. "Jack Rose") could have been far different from how the person was known previously. I have just had to give up hope of tracing the identity / origins of many individuals I am researching who arrived in England from eastern Europe.