Have you read the Survival guide
for the forum?
Sorry if you tried and came up empty, but we encourage everyone to try a search before asking a question. This particular question comes up very frequently.
A search for "Replace paragraph marks" gives me this thread (among others): Removing Paragraph per line of text
The bottom line is that OOo Writer does not handle this common task very easily or well, but it can be done with a little effort.
If you can't find what you need in that thread, just ask here.