We have good news for owners of Samsung's 2014 flagship as Galaxy S5s in Sri Lanka just got updated to Android Marshmallow. The S5 had already been updated in South Korea, Trinidad and Tobago and Argentina among other nations but this time it was the most common version of the S5 that started receiving the upgrade. The SM-G900F is the most commonly found version of the Galaxy S5 and is used primarily in most of Europe. This probably means that the remaining geographical locations where people use the SM-G900F will also start receiving the upgrade very soon.
Got o Settings, find About device and tap on it, select Software Update and check to see if the Marshmallow update has reached your smartphone yet. Note that this will NOT hasten the process, as the update will only arrive on your S5 when it's scheduled to arrive. It may give you some peace of mind though.
After it does arrive on your S5, you can schedule the entire download and set it up to be completed at a time that is convenient for you. On completion, your Android version will change to 6.0.1, PDA number will change to G900FXXS1CPC8 and CSC number will upgrade to G900FSLK1CPB2.