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The available thermal information for those devices are summarized in Table 7.4 (TPS62065-Q1) and Table 6.4 (TPS62590-Q1) of the respective datasheets.
You can refer as well to the following app note for more information on how to correctly apply those metrics to estimate the thermal performance of the device at you application conditions.
I'm not sure what you're trying to calculate, here are a few comments from my side, let me know if this helps.
By measuring/simulating the converter's efficiency at your application conditions you can estimate the power loss Ploss (W). Looking then at the package area A (m2, e.g. A = 2.0mm x 2.0 mm for TPS62065-Q1) you can derive the power loss density Ploss/A (W/m2)
Now, where and how efficiently this lost power is dissipated, and what is, for example, the resulting increase in junction temperature of the device, will depend on a lot of factors (package thermal performance, PCB layout, etc.), and is described in details in the app note I linked to above, using the device-specific thermal parameters listed in the respective datasheets.
