Your going to need a rifle clamp or Sandbags to hold the rifle steady.
With the scope mounted on the rifle ( Look up the answerto "How do you scope a rifle")
Load the rifle and set up a target at the distance you intend to shoot. ei 20 yards. Use a fairly large target. I use a large cardboard box filled with paper.
Using a sandbag or rifle clamp, aim the rifle at the target, clamp or sandbag the rifle as best you can and fire your first shot. (DO not move the rifle after the shot is fired. (It will jump a little) look though the scope and use the two adjusting knobs on the scope to adjust the cross-hairs to the impact point not the center of the target.
Now reload the rifle again and do the same thing over again. This time aim at the target center. Sandbag and fire your second shot. Make any correction to the new impact point. The new impact point should be closer to the target center. Remember do not move the rifle after the shot, keep it steady and make the adjustments. Then you can move the rifle to reload it.
It may take you 4 or 5 times to get it centered but it will happen. I always take a black marker and mark my impact point after each shot so I don't confuse it with the new impact point from the pellet.
Suggestions. USE good quality pellets. Don't use lightweight pellets like the Raptors. lightweight pellets wonder off target in a slight breeze. The heaver the pellet the straighter it travels. I recommend H&N pellets or Beeman target pellets.
The easy way is to go out and purchase a laser scope site. The laser fits into the barrel and projects a laser light on the target where the barrel is aimed. Then all you have to do is adjust you scope to the laser dot on the target. But that's too easy.
See link below-
The value of a Tasco 4x32 scope will depend on its condition, age, and specific model. On average, you can expect to pay between $30 to $70 for a used Tasco 4x32 scope. Newer or higher-quality models may command a higher price.
The age of an ACOG 4x32 scope is determined by the date of manufacture which is usually marked on the scope itself. Additionally, the model number and any specific features can also provide clues to when the scope was produced.
You can't it is not designed to come apart. Time for a new Scope.
No
About $30
Select an object about 50 yards away, then use both the front and rear ring to focus the object into view.
Send it back.
The typical Marlin 39A does not have a scope. They are drilled and tapped that one can be added.
5-100 usd
A brand new Beeman P1 sells for around $460. A used one in Excellent condition should sell for between $325 -$375. Without the Scope. Source Bluebook of airguns I have no price on the scope
It's worth between $10 to $15 used.