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.

There are two alternative options how to get the Premium:

If you have enough LoCoins you can purchase Premium for them - e.g. Locus translators and other supporters who get LC rewards. However, you have to activate this option first in the expert settings:
If your LoCoin balance is sufficiently high the LC purchase option activates in the offer:

If you need to avoid purchasing or subscribing to Premium via Google Play you can buy a one-year voucher at our website.

Our company gives away vouchers on various occasions, e.g. to supported Search&Rescue organizations.

Open your Locus account and tap Redeem voucher:

Open the app main menu > Go Premium > Select Premium plan. The voucher code is inserted here:
After inserting the voucher the app directly re-initializes in the Premium mode. When the voucher expires the app returns to its basic mode.