Visually select a range of words and press J to join them:

1. select 3 lines and Join them

VjjjJ

2. select a paragraph (text separated by an empty line(s)) and Join them

vipJ

Or for the whole buffer you can simply :%join or :%j to join all lines.

Visually select a range of words and press J to join them:

1. select 3 lines and Join them

VjjJ

2. select a paragraph (text separated by an empty line(s)) and Join them

vipJ

Or for the whole buffer you can simply :%join or :%j to join all lines.