Find - Replace command in MS Word is a great tool for such things.
As each line ends with a paragraph mark (EOL character), you can concatenate (join) the lines using the Find - Replace command.
For applying the command throughout the document just use the command below. For only a selection of text, first select the text and then use the command below.
Open the Find dialog box, in Find field type "^p" (^p for paragraph character) and in Replace field " " (a single space). Do a Replace All or Replace for each Find.
After this to remove extra spaces keeping only one space between words,
In Find field type " " (two spaces) and in Replace field " " (one space) and then do a Replace ALL a few times till the dialog box says "0 replacements".
This trick can be used to remove manual line breaks (^l), manual page breaks (^m), tab character (^t), non breaking space (^s), non breaking hiphen (^~), section break (^b) etc. in one go with Replace All command.