Assuming they are existing files generated by syslog (e.g. log files in text format with timestamps and all that), you should be able to just open the "Add Data" wizard and select either "Upload" or "Monitor" depending on where the existing files are located.
Follow both the wizard and the documentation and you should be good to go.
For instance, if you do "Upload" because the file is on your OWN computer, the docs will take you to the Upload Data page, then the Set Sourcetype page and so on.
Just be careful to click the links at the bottom of each "text" like
If you have further questions or need more help, either tack on more here (if it's about ingesting this specifically) or create a new question (if it's about something else regarding this data) and we'll try to help!