Hi Juniajon,
One option would be to use the Tools - Protection - Protect Sheet option.
Highlight the whole sheet and click into 'Format cells', go to the 'Protection' tab and make sure that the 'Locked' box is checked. This protects all the cells on the sheet.
The next step is to highlight the specific cells that you want updating, go back into 'Format cells' and uncheck the 'Locked' box. You could also colour these cells so it's really obvious which ones are to be for data entry.
The last step is to go to 'Tools' - 'Protection' - 'Protect sheet' and click OK. By default the Select locked cells and Select unlocked cells should be ticked; you can also enter a password at this screen if you want.
This has now locked all the cells on the sheet, except the ones you selected, you can move around these by pressing the Tab button.
As a thought, if you know in advance the specific data you want entered in the cells, you could use the 'Data' - 'Validation' option. In the 'Settings' tab choose Allow: List from the drop down box and select the range where you've entered the specific data as the 'Source'. When you now click in this cell you should get a drop down box with the available options.
I hope this was of use.