If you have recorded data, such as strip chart for weighing raingauge, and the dara was reported as hourly, you may be able to go back and reanalyze, estimating 15 minute intervals   But if you have a tipping bucket raingauge, set to report every hour, there is no way that I know of to get 15 minute detail.  

Aggregating hourly to daily data in MS Excel is fairly easy, as in your initial column, you have hourly readings, and next column add the 24 hourly cells into a one day cell in second column.  You may want to add a column or two at beginning with date time, and a day column before your daily readings that added the 24 hourly readings.  Excel lets you grab the corner of the cell with left mouse side held down, and replicate formulas as you pull it down column - see Excel tools or find someone familiar with Excel.  If you absolutely cannot find someone, let me know and I will make up a short example.

Dear William

Thanks for your answer. I would like to say that aggregation in excel, I am fine with it. I am looking for some code and tool available to do that. 

We have the tipping bucket and data that it stores are random mean whenever there will be rainfall it records. There is no option to segregate it into desired interval.

Can you scan in a strip chart, or digital example of how it collects data.  If digital, perhaps ypu can change for future readings to 5 minute or 15 minute intervals.  Strip charts are difficult to read to fine details, and there is a degree of visual interpretation.  I have heard of strip chart scanning for analysis, and have not ever tried that.  If you find a solution that is workable for you, then no need to send.

With R Studio. you can do what ever you want. I suggest hydroTSM package. Also aggregate function could be helpful.