I want to upgrade my Cisco NSO which is in 4.2.3 version to a 5.3 version. I have been looking for the upgrade path that I need to take but I didn't find any information. Is there any Cisco tools that can give me this information or can someone please give me the upgrade path.
The CDM migration documentation  states that upgrades from NSO releases 4.x.* to 5.*, where x>= 5, are supported.
The change of internal representation for leaf-lists was also a big change which needs to be taken into account.  The recommendation is to upgrade to NSO 4.5.* where the internal representation changed, and then upgrade to 5.* from there. (Note that manual work might be needed when upgrading to NSO 4.5.)
There are however many changes and fixes for leaf-lists after the NSO 4.5 release. See previous discussion for more information. 
Thus, a supported upgrade path would be:
4.2.3 -> 4.[5,6,7].* -> 5.3
However, with support from the reasoning above, I would recommend the following upgrade path:
Depending on which versions of NEDs you have you may or may not need to migrate them to later versions.
If you have fairly new NEDs you can probably build them for 4.5, 4.7 and 5.3. I would recommend that you upgrade the NEDs to latest version while on NSO 4.5/4.7 and then continue with the NSO migration to 5.3.
The NSO migration path is a subject that will be touched in Developer Days (in September this year)
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