can I use a 5% tolerance resistor instead of a 10% tolerance resistor? It is for an organ from the 60's. All resistors stated in Schematics are 10% 1/2 watt. But I can't find a 6.8k 1/2watt 10% tolerance resistor anywhere. All the resistors in the organ are carbon based.
Yes, so long as all the other specifications match.
"10% tolerance" on a component simply means that the actual value of the resistor must be within 10% of the specified value. That is, if you measured the actual resistance of a 6.8kΩ ± 10% resistor with a high-precision ohmmeter, you would measure its value as being somewhere between 6.12kΩ and 7.48kΩ.
As such, a 5% tolerance component will also fit the 10% specification: if the value is within 5%, it will certainly be within 10% as well. The lower tolerance just means that its value will be a more exact fit than the original design required!
Yes, you can, if that is the only difference.
Composition resistors are a bit different from more modern carbon film resistors- they are more tolerant of surges, for example.
If the resistors are of the same construction and of the same power rating (and of similar physical size- related to voltage and power rating) they will almost certainly work as well as the original. Voltage rating is not usually an issue on a product like an organ unless the resistor is connected to the mains. The same is true of surge capability (expand that to mains or power supply use). Don't replace a large resistor with a much smaller resistor (physical size) if you don't know the power rating.
Nominally, assuming all other things equal, the answer is yes, as other folks have pointed out.
It is worth noting that the distribution of discretes are 'notched' because of binning.
That is, the value of a 10% resistor will lie in the range \$[-10,-5) \cup (5,10]\%\$ because the 5% (and lower) will have been selected out after manufacturing.
Hence, while extremely unlikely in practice (and certainly if manufactured in the '60s), it is possible that a design may not work to specification if higher tolerance components are used.