You need to perform the upgrade in two main steps, divided in several sub-tasks:
1) 3.7 -> 2009 R2: you can find the upgrade toolkit in NAV 2009 DVD UpgradeToolKit folder, but better looking on PartnerSource for an updated document.
2) 2009 R2 -> 2013 R2: follow this toolkit guide.
Comments
1) 3.7 -> 2009 R2: you can find the upgrade toolkit in NAV 2009 DVD UpgradeToolKit folder, but better looking on PartnerSource for an updated document.
2) 2009 R2 -> 2013 R2: follow this toolkit guide.