insert or update row based on timestamp

Question

I'm looking for a solution in postgresql to insert or update a row, based on the timestamp of that row.

What I'm looking for is the following:

I want to insert the use of a specific hashtag, and count this use, based on the last hour.

+--------------------------------------------------+
| hashtag     | count | timestamp                  |
+--------------------------------------------------+
| hello       | 1     | 2020-01-01 12:00:00+00.00  |
| world       | 3     | 2020-01-01 12:00:00+00.00  |
| hello       | 2     | 2020-01-01 11:00:00+00.00  |
| ...                                              |
+--------------------------------------------------+

What I want to achieve:

I want to insert a new row if there is no row for the specific hashtag in the last hour created (based on the timestamp). If a row exists with timestamp > NOW() - INTERVAL '1 HOUR', I just want to update the count of that row.

How can I achieve this? Can I do this with a CHECK-constraint?