When may I lift or mark the player's ball?

Without asking the player first:

• when the ball is on the putting green: you may mark it, lift it, clean it and replace it, and
• when it's reasonable to conclude (from what the player says or does) that the player is going to take relief under a Rule.

Only with the player's authorisation (given each time, not once for the whole round):

• lifting the ball anywhere other than the putting green under a Rule that requires it to be replaced (for example, to identify it or because it's interfering with play).

Replacing: you may replace the ball only if you lifted or moved it.

Dropping: you may never drop the ball for the player; the player must drop it themselves.

Penalty: if you lift the ball when you weren't allowed to, the player gets one penalty stroke and the ball must be replaced.

Source: Rules of Golf (2023), Rules 10.3b, 14.1b, 14.2b and 14.3b.