Is there a way to change 3e stats into 5e stats?

Question

Is there any way (preferably and online generator or something akin to that, but I can do it by hand too), to quickly and hopefully simply, change a stat block like

Medium Aberration
HP 8d8+8 (44 hp)
Initiative +6 (+2 Dex, +4 Improved Initiative)
Speed 30 ft.
Armor Class 15 (+2 Dex, +3 natural)
Base Attack/Grapple: +6/+7
Attack: Tentacle +8 melee (1d4+1)
Full Attack: 4 tentacles +8 melee (1d4+1)
Space/Reach: 5 ft./5 ft.
Special Attacks: Mind blast, psionics, improved grab, extract
Special Qualities: Spell resistance 25, telepathy 100 ft.
Saves: Fort +3, Ref +4, Will +9
Abilities: Str 12, Dex 14, Con 12, Int 19, Wis 17, Cha 17
Skills: Bluff +11, Concentration +11, Diplomacy +7, Disguise +3 (+5 acting), Hide +10, Intimidate +9, Knowledge (any) 12, Listen +11, Move Silently +10, Sense Motive +7, Spot +11
Feats: Combat Casting, Improved Initiative, Weapon Finesse
Climate/Terrain: Underground
Orgnization: Solitary, pair, inquisition (3-5), or cult (3-5 plus 6-10 grimlocks)
Challenge Rating: 8
Treasure: Double standard
Alignment: Usually lawful evil
Advancement: By character class
Level Adjustment: +7
Source

Into a stat block like this

Medium aberration, lawful evil
Armor Class 15 (breastplate)
Hit Points 71 (13d8 + 13)
Speed 30 ft.
Str 11 (+0) | Dex 12 (+1) | Con 12 (+1), | Int 19 (+4) | Wis 17 (+3) | Cha 17 (+3)
Saving Throws lnt +7, Wis +6, Cha +6
Skills Arcana +7, Deception +6, Insight +6, Perception +12, Persuasion +6, Stealth +4
Senses darkvision 120ft., passive Perception 16
Languages Deep Speech, Undercommon, telepathy 120 ft.
Challenge 7 (2,900 XP)
Magic Resistance. The mind flayer has advantage on saving throws against spells and other magical effects.
Innate Spellcasting (Psionics). The mind flayer’s innnate spellcasting ability is intelligence (spell save DC 15). ITT can innately cast the following spells, requiring no concentration: At will - deter thoughts, levitate. 1/day each: dominate monster, plane shift (self only) Tentacles. Melee Weapon Attack: +7 to hit, reach 5ft., one target. Hit 15 (2d10+4) psychic damage. If the target is medium or smaller, it is grappled (East ape DC 15) and must succeed on a DC 15 Intelligence saving throw or be stunned until this grapple ends. Extract Brain. Melee Weapon Attack: +7 to hit, reach 5 ft., one incapacitated humanoid grappled by the min flayer. Hit: 55 (10d10) piercing damage. If this damage reduces the target to 0 hit points, the mind flayer kills the target by extracting and devouring its brain. Mind Blast (Recharge 5-6). The mind flayer magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 15 Intelligence saving throw or take 22 (4d8+4) psychic damage add be stunned for one minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Source

If not, are there any non-quick and easy ways?

Answer 1

The answer in this question has a useful link from Wizards of the Coast. This document contains guidelines on converting to 5e, including a "Quick Conversions" section on the last page, with a subsection for Monsters. For third edition monsters, that section includes:

Most statistics in third edition include the creature’s ability scores. Use the following parameters:

• Armor Class can be an average of touch AC and actual AC, or 20 percent lower than in third edition. The upper limit is 22.
• Attack roll modifiers are the appropriate ability score modifier + 3.
• Saving throw DCs are 10 + the appropriate ability score modifier.
• If a creature has to make a check or saving throw, use its ability score modifiers. Grant it a +3 bonus if it should be good at the roll.

Good luck.

It's worth noting that while ability scores in 3e tend to be higher than the 5e equivalents, the quick conversions only grant a +3 proficiency bonus. This will generally keep attacks for high CR creatures near sane levels, though do not be afraid to hard-cap attacks at +12 (the DMG recommended attack bonus for a Challenge 20 monster), at least for non-epic foes.