Turn off the power, remove the old bathroom ceiling light, then fit and wire in its replacement safely.
Bathroom light fittings must meet the IP rating for their zone under BS 7671 Part 701 - normally IP44 or higher in zones 0-2. A like-for-like swap itself doesn't need Part P notification in England, but the IP rating rule still applies.
Part P (England) doesn't require notification for a like-for-like light replacement. Adding a new circuit, moving the fitting, or adding a switch does need notifying or work by a registered electrician.
Before you start: Which electrical jobs can I do myself? Part P explained
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