After backup the database, can we delete older records?

You can certainly delete old historic data entries, but bear in mind these entries will no longer be available for graphs, tables, or reports. The best practice in such a case would be to stop PRTG, back up the necessary directories, delete the same, then restart PRTG.

Are you saying that I cannot archive them offline, and if needed, import back into PRTG in the future?

Btw, we wish to do partial archival, not total archival. How to go about this?

If you restore the database as it was, at a later time, you would of course once again have access to the historic data. In case of a partial back up, you would only delete the directories up to the point in time where you wish to retain your historic data.

Will you be able to list all the steps for backup/delete partial data, and restoration? I am new to this and needed detailed guideline.

I'm afraid there are no specific steps except those already mentioned. Stop PRTG, delete the directories in case (these are named according to the date of the monitored data, as outlined under How and where does PRTG store its data?), then start PRTG anew. To restore the directories, merely stop PRTG, copy the backed up directories into the database directory, and restart PRTG.

I'm trying to use this method to restore historic data from sensors which were part of a group that were acidentily deleted.

I have restored the configuration file and the sensors now exist and are collecting data. I have also restored all missing files from the C:\ProgramData\Application Data\Paessler\PRTG Network Monitor location (I assume that they are purged when the sensors are deleted).

My backup is a few days old so there will be two days where there are no databses for the deleted sensors but apart from that should this work?

I have also renamed the PRTG Graph Data Cache.dat file in the hope that this will trigger rebuilding of the historic data when viewed in the web UI.

Please notice that you will need the dataset files that are located under \Monitoring Database in the data directory. If you recalculate the data cache (and the respective files are available in the monitoring database directory) you should be able to discern historic data except for those days for which no data is available. Notice that the easiest way to recalculate the data cache is using the option under the "Memory Usage" tab of the Server Administration tool.