1. Levels
In photoshop, select:
Enhance (from the top bar) > adjust lighting > levels
and the levels box will appear.
The input levels are the top slider and have a graph above them. There are three arrows you can move under the x axis. The far left one controls how dark the darkest colours are, the far right one how bright the brightest colours are, and the middle arrow the balance between the two.
Compare both pictures - move the two outside arrows to get the bright and dark areas how you want them, then tweak the middle one to reach the right balance.
Advanced tip: you can just alter these settings for each of the red, green and blue components and altering the middle arrow particularly removes colour casts.
After that a very gentle smart blur sometimes tidies the picture.
2. Restoring letters
Browned and smudged paper often leads to difficult letters. You can white the area out and then replace with a font, but here's a way of tidying the text.
Alter the levels as above to acheive optimal appearance (you will have likely done this already)
Choose the icon on the left side where the red circle is (sponge tool). That brings up the sponge tool bar on the top.
There are three tools - the sponge tool, the dodge tool and the burn tool
Sponge tool - either increases or decreases the saturation of the area brushed. You can see in the pic below how it virtually eliminated the brown. In this example, that's all that is really needed, but I've done the next two steps as well.
Dodge tool - makes things brighter. For lettering use this on 'highlights' with an exposure of 25%. It make the highlights brighter (so will leave the dark letters alone). Eseentially it eliminated light greys. Use it too much and you get raggy edged letters.
Burn tool - makes things darker. Set on 'shadows' with similar exposure. It'll leave the white and darken the darks, so particularly it'll make darh grey black.
You choose the desaturation first, then dodge, then burn usually. Dodge and burn on those settings are also good for adding depth to comic book figures too.
