You can use the logresolvemerge.pl tool (see FAQ-COM350). Either you change the LogFile Option in the .conf file, or (for a one-time run) use it as argument in an update.
LogFile="/path/to/logresolvemerge.pl access.log* |"
An example (on a debian based system) for a command line update:
/usr/lib/cgi-bin/awstats.pl -update -config="example.com" -LogFile="/usr/share/awstats/tools/logresolvemerge.pl /var/log/apache2/access.log* |"
Remark: It is possible, that you need to clean the existing statistic-data. See FAQ-COM500.