On your XAMPP control panel, next to apache, select the "Config" option and select the first file (httpd.conf):
there, look for the "listen" line (you may use the find tool in the notepad) and there must be a line stating "Listen 80". Note: there are other lines with "listen" on them but they should be commented (start with a #), the one you need to change is the one saying exactly "listen 80". Now change it to "Listen 1337".
Start apache now.
If the error subsists, it's because there's another port that's already in use. So, select the config option again (next to apache in your xampp control panel) and select the second option this time (httpd-ssl.conf):
there, look for the line "Listen 443" and change it to "Listen 7331".
Start apache, it should be working now.