Actually, you renew it after 2.5 years, not 2 years. The earliest you can apply is 28 days before you reach 30 months in the UK, and the latest you can apply is the expiry date of your BRP card.
So, you take the date you landed in the UK on your current visa and count 30 months from that date. If you arrived in the UK in July 2018, you will reach 2.5 years in January 2021... and you can apply 28 days before the date in January that you reach 30 months... so that will be sometime in December 2020.
The next application is FLR(M):
https://www.gov.uk/government/publications/apply-to-extend-stay-in-the-uk-as-a-partner-or-dependent-child-form-flrmCurrent fees are £1,033 for the visa and £1,000 for the IHS surcharge, though this may have increased by the time you can apply in December.
You apply online (the date you submit the online application is your official application date), pay for the visa, book a biometrics appointment and then upload your documents before attending the appointment. It can take several weeks to get an appointment, or you can pay a bit more (about £100) for an out-of-hours appointment which are more readily available. Once you have attended the appointment, it can take up to 8 weeks to receive the visa.
Alternatively, you can pay £800 extra for a premium appointment which should give a decision within 24 hours. However, as the current processing time is only up to 8 weeks, a lot of people don't think it's worth it, and they save the £800 for ILR, which can take up to 6 months to be processed.
If you do use the £800 premium service, I would recommend applying later (i.e. closer to your visa expiry date in 2021) to give yourself more time to apply for ILR in 2023.
