Use the chown command to change user and group ownership of files and directories.

# To change the owner user of a file/folder:
chown <user_name> <path/to/file>

# To change the owner user and group of a file/folder:
chown <user_name>:<group_name> <path/to/file>

# To recursively change the owner of a folder and its contents:
chown -R <user_name> <path/to/folder>

# To change the owner of a symbolic link:
chown -h <user_name> <path/to/symlink>

# To change the owner of a file/folder to match a reference file:
chown --reference=<path/to/reference_file> <path/to/file>

# Mass change ownership, including files and directories with spaces.
find . -uid 0 -print0 | xargs -0 chown foo:foo
# uid 0        root user
# foo:foo      user:group to make owner and group. 
# .            the current directory and below.
# -print0, -0  indicate that filenames and directories are terminated by a null 
#              character instead of by whitespace.