Are the keys sticking because of food or other dirt and it needs cleaning? Little may be possible but a search will take you to instructions and alternatives (including simple things like squirts of compressed air) you can try.

Or are keys sticking because it's old and springs/mechanisms may be worn out?

New keyboards of basic design can be found for less than $10. I know you said you don't want a new one but an old one with sticky keys may not be fixable and it isn't like trying to do so is worth much effort. A can of compressed air can be $5 and may do nothing, that's halfway toward the cost of a new replacement keyboard.

I'm sure if you check YouTube you could find a video about cleaning your keyboard. I know the Cleaning Videos are there, just not sure which type keyboard you have, not all keyboards are created equal (many different way to clean and/or remove keys for cleaning)!

BTW, Elmer is right, your keyboard may just be worn out (it does happen). Your only option then would be to replace the keyboard with a new keyboard.

thanks