There are several options to open it in the app, choose which suits you the best:

The primary link. You can find it in the main menu:

They are scattered all over the free app, for example here:
Tapping such a feature opens a box with detailed information about it:
You can either close the box or continue…

Browse the options or tap the selection button right away:
After signing up or in the Locus account, choose your plan - Silver or Gold. Premium annual subscription is the default choice:
You can also select other options - shorter subscription or one-time payments:

All payments are processed by Google Play with the options available in your country.

When your Premium plan is active you have a few options for the plan transfer, see >>

There are two alternative options for getting the Premium:

If you have LoCoins